[CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH #176420

aengelke · 2026-01-16T16:20:01Z

This patch implements PCH support. PCH is enabled by default, unless noted below, and can be disabled with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON.

Libraries can defined precompiled headers using a newly added PRECOMPILE_HEADERS keyword. If specified, the listed headers will be compiled into a pre-compiled header using standard CMake mechanisms.
Libraries that don't define their own PRECOMPILE_HEADERS but directly depend on a library or component that defines its own PCH will reuse that PCH. This reuse is not transitive to prevent excessive use of unrelated headers. If multiple dependencies provide a reusable PCH, the first one with the longest dependency chain (stored in the CMake target property LLVM_PCH_PRIORITY) is used. However, due to CMake limitations, only PCH from targets that are already defined can be reused; therefore libraries that should reuse a PCH must be defined later in the CMake file (=> add_subdirectory order matters).
Libraries and executables can prevent PCH reuse with the keyword DISABLE_PCH_REUSE. This both prevents reuse from dependencies and reuse by other dependants. This is useful when, e.g., internal headers are used in the PCH or the used headers are unlikely to provide benefits for dependants.
Precompiled headers are only used for C++ targets. Due to CMake-weirdness, C-only targets must explicitly disable PCH reuse, otherwise configuration fails.
With GCC, PCH provide very little benefits (tested with GCC 14 and 15) due to increased template instatiation costs, but substantially increase max-rss and build directory size. Therefore, disable PCH with GCC by default; this can be explicitly overridden on the command line with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF.
With ccache and non-Clang compilers, changes in macro definitions are not always accurately forwarded with ccache's preprocessed mode. To be on the safe side, when ccache is enabled, disable PCH with all non-Clang compilers; this can be explicitly overridden.
Add a base PCH to LLVMSupport, which includes widely used standard library and Support+ADT headers. The pch.h is placed in include so that later PCH headers can extend that list of headers.
Flang PCH use is ported to the general mechanism.

Addition of PCH headers for other components (e.g., IR, CodeGen) will be posted as separate PRs.

RFC: https://discourse.llvm.org/t/rfc-use-pre-compiled-headers-to-speed-up-llvm-build-by-1-5-2x/89345

This patch implements PCH support. * Libraries can defined precompiled headers using a newly added PRECOMPILE_HEADERS keyword. If specified, the listed headers will be compiled into a pre-compiled header using standard CMake mechanisms. * Libraries that don't define their own PRECOMPILE_HEADERS but directly depend on a library or component that defines its own PCH will reuse that PCH. This reuse is not transitive to prevent excessive use of unrelated headers. If multiple dependencies provide a reusable PCH, the first one with the longest dependency chain (stored in the CMake target property LLVM_PCH_PRIORITY) is used. However, due to CMake limitations, only PCH from targets that are already defined can be reused; therefore libraries that should reuse a PCH must be defined later in the CMake file (=> add_subdirectory order matters). * Libraries and executables can prevent PCH reuse with the keyword DISABLE_PCH_REUSE. This both prevents reuse from dependencies and reuse by other dependants. This is useful when, e.g., internal headers are used in the PCH or the used headers are unlikely to provide benefits for dependants. * Precompiled headers are only used for C++ targets. Due to CMake-weirdness, C-only targets must explicitly disable PCH reuse, otherwise configuration fails. * With GCC, PCH provide very little benefits (tested with GCC 14 and 15) due to increased template instatiation costs, but substantially increase max-rss and build directory size. Therefore, disable PCH with GCC by default; this can be explicitly overridden on the command line with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF. * With ccache and non-Clang compilers, changes in macro definitions are not always accurately forwarded with ccache's preprocessed mode. To be on the safe side, when ccache is enabled, disable PCH with all non-Clang compilers; this can be explicitly overridden. * Add a base PCH to LLVMSupport, which includes widely used standard library and Support+ADT headers. The pch.h is placed in include so that later PCH headers can extend that list of headers. * Flang PCH use is ported to the general mechanism. Addition of PCH headers for other components (e.g., IR, CodeGen) will be posted afterwards as separate PRs. RFC: https://discourse.llvm.org/t/rfc-use-pre-compiled-headers-to-speed-up-llvm-build-by-1-5-2x/89345

llvmbot · 2026-01-16T16:20:36Z

@llvm/pr-subscribers-infrastructure
@llvm/pr-subscribers-flang-parser
@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-flang-driver

Author: Alexis Engelke (aengelke)

Changes

This patch implements PCH support.

Libraries can defined precompiled headers using a newly added PRECOMPILE_HEADERS keyword. If specified, the listed headers will be compiled into a pre-compiled header using standard CMake mechanisms.
Libraries that don't define their own PRECOMPILE_HEADERS but directly depend on a library or component that defines its own PCH will reuse that PCH. This reuse is not transitive to prevent excessive use of unrelated headers. If multiple dependencies provide a reusable PCH, the first one with the longest dependency chain (stored in the CMake target property LLVM_PCH_PRIORITY) is used. However, due to CMake limitations, only PCH from targets that are already defined can be reused; therefore libraries that should reuse a PCH must be defined later in the CMake file (=> add_subdirectory order matters).
Libraries and executables can prevent PCH reuse with the keyword DISABLE_PCH_REUSE. This both prevents reuse from dependencies and reuse by other dependants. This is useful when, e.g., internal headers are used in the PCH or the used headers are unlikely to provide benefits for dependants.
Precompiled headers are only used for C++ targets. Due to CMake-weirdness, C-only targets must explicitly disable PCH reuse, otherwise configuration fails.
With GCC, PCH provide very little benefits (tested with GCC 14 and 15) due to increased template instatiation costs, but substantially increase max-rss and build directory size. Therefore, disable PCH with GCC by default; this can be explicitly overridden on the command line with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF.
With ccache and non-Clang compilers, changes in macro definitions are not always accurately forwarded with ccache's preprocessed mode. To be on the safe side, when ccache is enabled, disable PCH with all non-Clang compilers; this can be explicitly overridden.
Add a base PCH to LLVMSupport, which includes widely used standard library and Support+ADT headers. The pch.h is placed in include so that later PCH headers can extend that list of headers.
Flang PCH use is ported to the general mechanism.

Addition of PCH headers for other components (e.g., IR, CodeGen) will be posted as separate PRs.

RFC: https://discourse.llvm.org/t/rfc-use-pre-compiled-headers-to-speed-up-llvm-build-by-1-5-2x/89345

Patch is 25.98 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/176420.diff

25 Files Affected:

(modified) clang/tools/c-index-test/CMakeLists.txt (+2)
(modified) clang/tools/clang-fuzzer/dictionary/CMakeLists.txt (+2)
(modified) flang/CMakeLists.txt (-10)
(modified) flang/lib/Evaluate/CMakeLists.txt (+9-9)
(modified) flang/lib/Frontend/CMakeLists.txt (+8-8)
(modified) flang/lib/Lower/CMakeLists.txt (+11-11)
(modified) flang/lib/Parser/CMakeLists.txt (+8-8)
(modified) flang/lib/Semantics/CMakeLists.txt (+9-9)
(modified) llvm/CMakeLists.txt (+16)
(modified) llvm/cmake/modules/AddLLVM.cmake (+69-8)
(modified) llvm/cmake/modules/HandleLLVMOptions.cmake (+21)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsRemovableCode/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsVeryLazy/CMakeLists.txt (+2)
(added) llvm/include/llvm/Support/pch.h (+75)
(modified) llvm/lib/CMakeLists.txt (+1-1)
(modified) llvm/lib/Support/CMakeLists.txt (+6-1)
(modified) llvm/tools/llvm-c-test/CMakeLists.txt (+2)
(modified) llvm/utils/count/CMakeLists.txt (+2)
(modified) mlir/test/CAPI/CMakeLists.txt (+1)

diff --git a/clang/tools/c-index-test/CMakeLists.txt b/clang/tools/c-index-test/CMakeLists.txt
index 41e80e66ffa7a..2bbd387d6e812 100644
--- a/clang/tools/c-index-test/CMakeLists.txt
+++ b/clang/tools/c-index-test/CMakeLists.txt
@@ -5,6 +5,8 @@ set(LLVM_LINK_COMPONENTS
 add_clang_executable(c-index-test
   c-index-test.c
   core_main.cpp
+
+  DISABLE_PCH_REUSE # Prevent CMake error with C source files.
   )
 
 if(NOT MSVC)
diff --git a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
index 6b72b98f5e1c4..a7f18965b4e29 100644
--- a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
+++ b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
@@ -1,5 +1,7 @@
 set(CMAKE_CXX_FLAGS ${CXX_FLAGS_NOFUZZ})
 add_clang_executable(clang-fuzzer-dictionary
   dictionary.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
 
diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index c01eb56d5e496..e21304d2e4da7 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -441,11 +441,6 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   if (BUILD_SHARED_LIBS AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "Clang"))
    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-semantic-interposition")
   endif()
-
-  # GCC requires this flag in order for precompiled headers to work with ccache
-  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpch-preprocess")
-  endif()
 endif()
 
 # Clang on Darwin enables non-POSIX extensions by default, which allows the
@@ -456,11 +451,6 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
-# Clang requires this flag in order for precompiled headers to work with ccache
-if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
-endif()
-
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.
diff --git a/flang/lib/Evaluate/CMakeLists.txt b/flang/lib/Evaluate/CMakeLists.txt
index 24a1c9004bc3b..472ecb6d8d079 100644
--- a/flang/lib/Evaluate/CMakeLists.txt
+++ b/flang/lib/Evaluate/CMakeLists.txt
@@ -66,15 +66,8 @@ add_flang_library(FortranEvaluate
   ${LIBPGMATH}
   ${QUADMATHLIB}
 
-  LINK_COMPONENTS
-  Support
-
-  DEPENDS
-  acc_gen
-  omp_gen
-)
-
-target_precompile_headers(FortranEvaluate PRIVATE
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
   [["flang/Evaluate/common.h"]]
   [["flang/Evaluate/call.h"]]
   [["flang/Evaluate/traverse.h"]]
@@ -86,4 +79,11 @@ target_precompile_headers(FortranEvaluate PRIVATE
   [["flang/Evaluate/integer.h"]]
   [["flang/Evaluate/expression.h"]]
   [["flang/Evaluate/tools.h"]]
+
+  LINK_COMPONENTS
+  Support
+
+  DEPENDS
+  acc_gen
+  omp_gen
 )
diff --git a/flang/lib/Frontend/CMakeLists.txt b/flang/lib/Frontend/CMakeLists.txt
index 4ebe497e65676..bd67ec444f358 100644
--- a/flang/lib/Frontend/CMakeLists.txt
+++ b/flang/lib/Frontend/CMakeLists.txt
@@ -13,6 +13,14 @@ add_flang_library(flangFrontend
   TextDiagnosticBuffer.cpp
   TextDiagnostic.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/dump-parse-tree.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/Bridge.h"]]
+
   DEPENDS
   CUFDialect
   FIRDialect
@@ -78,11 +86,3 @@ add_flang_library(flangFrontend
   clangBasic
   clangOptions
 )
-
-target_precompile_headers(flangFrontend PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/dump-parse-tree.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/Bridge.h"]]
-)
diff --git a/flang/lib/Lower/CMakeLists.txt b/flang/lib/Lower/CMakeLists.txt
index 230a56ab66ec5..f5424c78b9a98 100644
--- a/flang/lib/Lower/CMakeLists.txt
+++ b/flang/lib/Lower/CMakeLists.txt
@@ -38,6 +38,17 @@ add_flang_library(FortranLower
   Support/Utils.cpp
   SymbolMap.cpp
   VectorSubscripts.cpp
+
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Lower/ConvertExpr.h"]]
+  [["flang/Lower/SymbolMap.h"]]
+  [["flang/Lower/AbstractConverter.h"]]
+  [["flang/Lower/IterationSpace.h"]]
+  [["flang/Lower/CallInterface.h"]]
+  [["flang/Lower/BoxAnalyzer.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/DirectivesCommon.h"]]
   
   DEPENDS
   CUFAttrs
@@ -79,14 +90,3 @@ add_flang_library(FortranLower
   MLIRLLVMDialect
   MLIRSCFToControlFlow
 )
-
-target_precompile_headers(FortranLower PRIVATE
-  [["flang/Lower/ConvertExpr.h"]]
-  [["flang/Lower/SymbolMap.h"]]
-  [["flang/Lower/AbstractConverter.h"]]
-  [["flang/Lower/IterationSpace.h"]]
-  [["flang/Lower/CallInterface.h"]]
-  [["flang/Lower/BoxAnalyzer.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/DirectivesCommon.h"]]
-)
diff --git a/flang/lib/Parser/CMakeLists.txt b/flang/lib/Parser/CMakeLists.txt
index 20c6c2a7c8f80..e0479b0da3eb8 100644
--- a/flang/lib/Parser/CMakeLists.txt
+++ b/flang/lib/Parser/CMakeLists.txt
@@ -25,6 +25,14 @@ add_flang_library(FortranParser
   unparse.cpp
   user-state.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/provenance.h"]]
+  [["flang/Parser/message.h"]]
+  [["flang/Parser/parse-tree-visitor.h"]]
+
   LINK_LIBS
   FortranSupport
 
@@ -37,11 +45,3 @@ add_flang_library(FortranParser
   omp_gen
   acc_gen
 )
-
-target_precompile_headers(FortranParser PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/provenance.h"]]
-  [["flang/Parser/message.h"]]
-  [["flang/Parser/parse-tree-visitor.h"]]
-)
diff --git a/flang/lib/Semantics/CMakeLists.txt b/flang/lib/Semantics/CMakeLists.txt
index 109bc2dbb8569..44e6dfb4dd09f 100644
--- a/flang/lib/Semantics/CMakeLists.txt
+++ b/flang/lib/Semantics/CMakeLists.txt
@@ -53,6 +53,15 @@ add_flang_library(FortranSemantics
   type.cpp
   unparse-with-symbols.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Semantics/semantics.h"]]
+  [["flang/Semantics/type.h"]]
+  [["flang/Semantics/openmp-modifiers.h"]]
+  [["flang/Semantics/expression.h"]]
+  [["flang/Semantics/tools.h"]]
+  [["flang/Semantics/symbol.h"]]
+
   DEPENDS
   acc_gen
   omp_gen
@@ -68,12 +77,3 @@ add_flang_library(FortranSemantics
   FrontendOpenACC
   TargetParser
 )
-
-target_precompile_headers(FortranSemantics PRIVATE
-  [["flang/Semantics/semantics.h"]]
-  [["flang/Semantics/type.h"]]
-  [["flang/Semantics/openmp-modifiers.h"]]
-  [["flang/Semantics/expression.h"]]
-  [["flang/Semantics/tools.h"]]
-  [["flang/Semantics/symbol.h"]]
-)
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 9954053591d30..a539a2182efa5 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -313,6 +313,22 @@ if(LLVM_CCACHE_BUILD)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
+
+      if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        # ccache with PCH can lead to false-positives when only a macro
+        # definition changes with non-Clang compilers, because macro definitions
+        # are not compared in preprocessed mode.
+        # See: https://github.com/ccache/ccache/issues/1668
+        if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(NOTICE "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported. CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON. "
+            "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override this.")
+          set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+        elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(WARNING "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported.")
+        endif()
+      endif()
     else()
       # Until a way to reliably configure ccache on Windows is found,
       # disable precompiled headers for Windows + ccache builds
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index d938214f9d0df..c9ed03d98cc9c 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -79,6 +79,48 @@ function(llvm_update_compile_flags name)
   target_compile_definitions(${name} PRIVATE ${LLVM_COMPILE_DEFINITIONS})
 endfunction()
 
+function(llvm_update_pch name)
+  if(LLVM_REQUIRES_RTTI OR LLVM_REQUIRES_EH)
+    # Non-default RTTI/EH results in incompatible flags, precluding PCH reuse.
+    set(ARG_DISABLE_PCH_REUSE ON)
+  endif()
+
+  # Find PCH with highest priority from dependencies. We reuse the first PCH
+  # with the highest priority. If the target has its own set of PCH, we give it
+  # a higher priority so that dependents will prefer the new PCH. We don't do
+  # transitive PCH reuse, because this causes too many unrelated naming
+  # collisions (e.g., in A -> B -> C{pch}, only B would reuse the PCH of C).
+  set(pch_priority 0)
+  llvm_map_components_to_libnames(libs
+    ${LLVM_LINK_COMPONENTS}
+  )
+  list(APPEND libs ${ARG_LINK_LIBS})
+  foreach(lib ${libs})
+    if(TARGET ${lib})
+      get_target_property(lib_pch_priority ${lib} LLVM_PCH_PRIORITY)
+      if(${lib_pch_priority} GREATER ${pch_priority})
+        set(pch_priority ${lib_pch_priority})
+        set(pch_reuse ${lib})
+      endif()
+    endif()
+  endforeach()
+
+  if(ARG_PRECOMPILE_HEADERS)
+    message(DEBUG "Adding PCH ${ARG_PRECOMPILE_HEADERS} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} PRIVATE $<$<COMPILE_LANGUAGE:CXX>:${ARG_PRECOMPILE_HEADERS}>)
+    if(NOT ARG_DISABLE_PCH_REUSE)
+      # Set priority so that dependants can reuse the PCH.
+      math(EXPR pch_priority "${pch_priority} + 1")
+      set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
+    endif()
+  elseif(pch_reuse AND NOT ARG_DISABLE_PCH_REUSE)
+    message(DEBUG "Using PCH ${pch_reuse} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} REUSE_FROM ${pch_reuse})
+  else()
+    message(DEBUG "Using NO PCH for ${name}")
+  endif()
+endfunction()
+
 function(add_llvm_symbol_exports target_name export_file)
   if("${CMAKE_SYSTEM_NAME}" MATCHES "Darwin")
     set(native_export_file "${target_name}.exports")
@@ -485,6 +527,13 @@ endfunction(set_windows_version_resource_properties)
 #     Corresponds to OUTPUT_NAME in target properties.
 #   DEPENDS targets...
 #     Same semantics as add_dependencies().
+#   PRECOMPILE_HEADERS include_directives...
+#     Pre-compiled C++ headers to use. PCH can be reused by dependants. If
+#     specified, no PCHs from dependencies will be reused.
+#   DISABLE_PCH_REUSE
+#     Disable reuse of pre-compiled headers in both directions: the library will
+#     not reuse the PCH of a dependency and a defined PCH will not be offered
+#     for reuse by dependants.
 #   LINK_COMPONENTS components...
 #     Same as the variable LLVM_LINK_COMPONENTS.
 #   LINK_LIBS lib_targets...
@@ -504,11 +553,12 @@ endfunction(set_windows_version_resource_properties)
 #   )
 function(llvm_add_library name)
   cmake_parse_arguments(ARG
-    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB"
+    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB;DISABLE_PCH_REUSE"
     "OUTPUT_NAME;PLUGIN_TOOL;ENTITLEMENTS;BUNDLE_PATH"
-    "ADDITIONAL_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
+    "ADDITIONAL_HEADERS;PRECOMPILE_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
     ${ARGN})
   list(APPEND LLVM_COMMON_DEPENDS ${ARG_DEPENDS})
+  list(APPEND LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS})
   if(ARG_ADDITIONAL_HEADERS)
     # Pass through ADDITIONAL_HEADERS.
     set(ARG_ADDITIONAL_HEADERS ADDITIONAL_HEADERS ${ARG_ADDITIONAL_HEADERS})
@@ -550,6 +600,7 @@ function(llvm_add_library name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     if(CMAKE_GENERATOR STREQUAL "Xcode")
       set(DUMMY_FILE ${CMAKE_CURRENT_BINARY_DIR}/Dummy.c)
       file(WRITE ${DUMMY_FILE} "// This file intentionally empty\n")
@@ -604,7 +655,7 @@ function(llvm_add_library name)
       ${output_name}
       OBJLIBS ${ALL_FILES} # objlib
       LINK_LIBS ${ARG_LINK_LIBS}
-      LINK_COMPONENTS ${ARG_LINK_COMPONENTS}
+      LINK_COMPONENTS ${LLVM_LINK_COMPONENTS}
       )
     set_target_properties(${name_static} PROPERTIES FOLDER "${subproject_title}/Libraries")
 
@@ -676,6 +727,11 @@ function(llvm_add_library name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT obj_name)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
   add_link_opts( ${name} )
   if(ARG_OUTPUT_NAME)
@@ -758,8 +814,7 @@ function(llvm_add_library name)
         target_compile_definitions(${name} PRIVATE LLVM_BUILD_STATIC)
       endif()
       llvm_map_components_to_libnames(llvm_libs
-       ${ARG_LINK_COMPONENTS}
-       ${LLVM_LINK_COMPONENTS}
+        ${LLVM_LINK_COMPONENTS}
        )
     endif()
   else()
@@ -770,7 +825,7 @@ function(llvm_add_library name)
     # It would be nice to verify that we have the dependencies for this library
     # name, but using get_property(... SET) doesn't suffice to determine if a
     # property has been set to an empty value.
-    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS} ${LLVM_LINK_COMPONENTS})
+    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${LLVM_LINK_COMPONENTS})
 
     # This property is an internal property only used to make sure the
     # link step applied in LLVMBuildResolveComponentsLink uses the same
@@ -993,6 +1048,7 @@ macro(generate_llvm_objects name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>")
     if(ARG_DEPENDS)
       add_dependencies(${obj_name} ${ARG_DEPENDS})
@@ -1032,7 +1088,7 @@ endmacro()
 
 macro(add_llvm_executable name)
   cmake_parse_arguments(ARG
-    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS"
+    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS;DISABLE_PCH_REUSE"
     "ENTITLEMENTS;BUNDLE_PATH"
     ""
     ${ARGN})
@@ -1073,6 +1129,11 @@ macro(add_llvm_executable name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
 
   if (ARG_SUPPORT_PLUGINS AND NOT "${CMAKE_SYSTEM_NAME}" MATCHES "AIX")
@@ -1780,7 +1841,7 @@ function(add_unittest test_suite test_name)
   endif()
 
   list(APPEND LLVM_LINK_COMPONENTS Support) # gtest needs it for raw_ostream
-  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH ${ARGN})
+  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH DISABLE_PCH_REUSE ${ARGN})
   get_subproject_title(subproject_title)
   set_target_properties(${test_name} PROPERTIES FOLDER "${subproject_title}/Tests/Unit")
 
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 311123084bf58..2e90c0ad0ad76 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -1304,6 +1304,27 @@ if (LLVM_BUILD_INSTRUMENTED AND LLVM_BUILD_INSTRUMENTED_COVERAGE)
   message(FATAL_ERROR "LLVM_BUILD_INSTRUMENTED and LLVM_BUILD_INSTRUMENTED_COVERAGE cannot both be specified")
 endif()
 
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  # Pre-compiled headers with GCC (tested versions 14+15) provide very little
+  # compile-time improvements, but substantially increase the build dir size.
+  # Therefore, disable PCH with GCC by default.
+  message(NOTICE "Precompiled headers are disabled by default with GCC. "
+    "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override.")
+  set(CMAKE_DISABLE_PRECOMPILE_HEADERS ON)
+endif()
+if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  message(STATUS "Precompiled headers enabled.")
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    # Clang requires this flag in order for precompiled headers to work with ccache
+    append("-Xclang -fno-pch-timestamp" CMAKE_CXX_FLAGS)
+  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # GCC requires this flag in order for precompiled headers to work with ccache
+    append("-fpch-preprocess" CMAKE_CXX_FLAGS)
+  endif()
+else()
+  message(STATUS "Precompiled headers disabled.")
+endif()
+
 set(LLVM_THINLTO_CACHE_PATH "${PROJECT_BINARY_DIR}/lto.cache" CACHE STRING "Set ThinLTO cache path. This can be used when building LLVM from several different directiories.")
 
 if(LLVM_ENABLE_LTO AND WIN32 AND NOT LINKER_IS_LLD_LINK AND NOT MINGW)
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
index cc50112f326ea..9a82cc0ed0916 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsAddObjectFile
   OrcV2CBindingsAddObjectFile.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
index 0f18d6c24912f..1a738f60d15e1 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsBasicUsage
   OrcV2CBindingsBasicUsage.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
index 8e2c97d782062..c7f748a12bf35 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsDumpObjects
   OrcV2CBindingsDumpObjects.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
index af1b43e43e111..b152faf1e3527 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
@@ -13,4 +13,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsIRTransforms
   OrcV2CBindingsIRTransforms.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
index 52eb2d496fc23..c46700d9b2576 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsLazy
   OrcV2CBindingsLazy.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
index 4cdc2b114e9a5..d4a06104120c8 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_exampl...
[truncated]

llvmbot · 2026-01-16T16:20:36Z

@llvm/pr-subscribers-llvm-support

Author: Alexis Engelke (aengelke)

Changes

This patch implements PCH support.

Libraries can defined precompiled headers using a newly added PRECOMPILE_HEADERS keyword. If specified, the listed headers will be compiled into a pre-compiled header using standard CMake mechanisms.
Libraries that don't define their own PRECOMPILE_HEADERS but directly depend on a library or component that defines its own PCH will reuse that PCH. This reuse is not transitive to prevent excessive use of unrelated headers. If multiple dependencies provide a reusable PCH, the first one with the longest dependency chain (stored in the CMake target property LLVM_PCH_PRIORITY) is used. However, due to CMake limitations, only PCH from targets that are already defined can be reused; therefore libraries that should reuse a PCH must be defined later in the CMake file (=> add_subdirectory order matters).
Libraries and executables can prevent PCH reuse with the keyword DISABLE_PCH_REUSE. This both prevents reuse from dependencies and reuse by other dependants. This is useful when, e.g., internal headers are used in the PCH or the used headers are unlikely to provide benefits for dependants.
Precompiled headers are only used for C++ targets. Due to CMake-weirdness, C-only targets must explicitly disable PCH reuse, otherwise configuration fails.
With GCC, PCH provide very little benefits (tested with GCC 14 and 15) due to increased template instatiation costs, but substantially increase max-rss and build directory size. Therefore, disable PCH with GCC by default; this can be explicitly overridden on the command line with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF.
With ccache and non-Clang compilers, changes in macro definitions are not always accurately forwarded with ccache's preprocessed mode. To be on the safe side, when ccache is enabled, disable PCH with all non-Clang compilers; this can be explicitly overridden.
Add a base PCH to LLVMSupport, which includes widely used standard library and Support+ADT headers. The pch.h is placed in include so that later PCH headers can extend that list of headers.
Flang PCH use is ported to the general mechanism.

Addition of PCH headers for other components (e.g., IR, CodeGen) will be posted as separate PRs.

RFC: https://discourse.llvm.org/t/rfc-use-pre-compiled-headers-to-speed-up-llvm-build-by-1-5-2x/89345

Patch is 25.98 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/176420.diff

25 Files Affected:

(modified) clang/tools/c-index-test/CMakeLists.txt (+2)
(modified) clang/tools/clang-fuzzer/dictionary/CMakeLists.txt (+2)
(modified) flang/CMakeLists.txt (-10)
(modified) flang/lib/Evaluate/CMakeLists.txt (+9-9)
(modified) flang/lib/Frontend/CMakeLists.txt (+8-8)
(modified) flang/lib/Lower/CMakeLists.txt (+11-11)
(modified) flang/lib/Parser/CMakeLists.txt (+8-8)
(modified) flang/lib/Semantics/CMakeLists.txt (+9-9)
(modified) llvm/CMakeLists.txt (+16)
(modified) llvm/cmake/modules/AddLLVM.cmake (+69-8)
(modified) llvm/cmake/modules/HandleLLVMOptions.cmake (+21)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsRemovableCode/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsVeryLazy/CMakeLists.txt (+2)
(added) llvm/include/llvm/Support/pch.h (+75)
(modified) llvm/lib/CMakeLists.txt (+1-1)
(modified) llvm/lib/Support/CMakeLists.txt (+6-1)
(modified) llvm/tools/llvm-c-test/CMakeLists.txt (+2)
(modified) llvm/utils/count/CMakeLists.txt (+2)
(modified) mlir/test/CAPI/CMakeLists.txt (+1)

diff --git a/clang/tools/c-index-test/CMakeLists.txt b/clang/tools/c-index-test/CMakeLists.txt
index 41e80e66ffa7a..2bbd387d6e812 100644
--- a/clang/tools/c-index-test/CMakeLists.txt
+++ b/clang/tools/c-index-test/CMakeLists.txt
@@ -5,6 +5,8 @@ set(LLVM_LINK_COMPONENTS
 add_clang_executable(c-index-test
   c-index-test.c
   core_main.cpp
+
+  DISABLE_PCH_REUSE # Prevent CMake error with C source files.
   )
 
 if(NOT MSVC)
diff --git a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
index 6b72b98f5e1c4..a7f18965b4e29 100644
--- a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
+++ b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
@@ -1,5 +1,7 @@
 set(CMAKE_CXX_FLAGS ${CXX_FLAGS_NOFUZZ})
 add_clang_executable(clang-fuzzer-dictionary
   dictionary.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
 
diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index c01eb56d5e496..e21304d2e4da7 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -441,11 +441,6 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   if (BUILD_SHARED_LIBS AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "Clang"))
    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-semantic-interposition")
   endif()
-
-  # GCC requires this flag in order for precompiled headers to work with ccache
-  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpch-preprocess")
-  endif()
 endif()
 
 # Clang on Darwin enables non-POSIX extensions by default, which allows the
@@ -456,11 +451,6 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
-# Clang requires this flag in order for precompiled headers to work with ccache
-if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
-endif()
-
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.
diff --git a/flang/lib/Evaluate/CMakeLists.txt b/flang/lib/Evaluate/CMakeLists.txt
index 24a1c9004bc3b..472ecb6d8d079 100644
--- a/flang/lib/Evaluate/CMakeLists.txt
+++ b/flang/lib/Evaluate/CMakeLists.txt
@@ -66,15 +66,8 @@ add_flang_library(FortranEvaluate
   ${LIBPGMATH}
   ${QUADMATHLIB}
 
-  LINK_COMPONENTS
-  Support
-
-  DEPENDS
-  acc_gen
-  omp_gen
-)
-
-target_precompile_headers(FortranEvaluate PRIVATE
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
   [["flang/Evaluate/common.h"]]
   [["flang/Evaluate/call.h"]]
   [["flang/Evaluate/traverse.h"]]
@@ -86,4 +79,11 @@ target_precompile_headers(FortranEvaluate PRIVATE
   [["flang/Evaluate/integer.h"]]
   [["flang/Evaluate/expression.h"]]
   [["flang/Evaluate/tools.h"]]
+
+  LINK_COMPONENTS
+  Support
+
+  DEPENDS
+  acc_gen
+  omp_gen
 )
diff --git a/flang/lib/Frontend/CMakeLists.txt b/flang/lib/Frontend/CMakeLists.txt
index 4ebe497e65676..bd67ec444f358 100644
--- a/flang/lib/Frontend/CMakeLists.txt
+++ b/flang/lib/Frontend/CMakeLists.txt
@@ -13,6 +13,14 @@ add_flang_library(flangFrontend
   TextDiagnosticBuffer.cpp
   TextDiagnostic.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/dump-parse-tree.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/Bridge.h"]]
+
   DEPENDS
   CUFDialect
   FIRDialect
@@ -78,11 +86,3 @@ add_flang_library(flangFrontend
   clangBasic
   clangOptions
 )
-
-target_precompile_headers(flangFrontend PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/dump-parse-tree.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/Bridge.h"]]
-)
diff --git a/flang/lib/Lower/CMakeLists.txt b/flang/lib/Lower/CMakeLists.txt
index 230a56ab66ec5..f5424c78b9a98 100644
--- a/flang/lib/Lower/CMakeLists.txt
+++ b/flang/lib/Lower/CMakeLists.txt
@@ -38,6 +38,17 @@ add_flang_library(FortranLower
   Support/Utils.cpp
   SymbolMap.cpp
   VectorSubscripts.cpp
+
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Lower/ConvertExpr.h"]]
+  [["flang/Lower/SymbolMap.h"]]
+  [["flang/Lower/AbstractConverter.h"]]
+  [["flang/Lower/IterationSpace.h"]]
+  [["flang/Lower/CallInterface.h"]]
+  [["flang/Lower/BoxAnalyzer.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/DirectivesCommon.h"]]
   
   DEPENDS
   CUFAttrs
@@ -79,14 +90,3 @@ add_flang_library(FortranLower
   MLIRLLVMDialect
   MLIRSCFToControlFlow
 )
-
-target_precompile_headers(FortranLower PRIVATE
-  [["flang/Lower/ConvertExpr.h"]]
-  [["flang/Lower/SymbolMap.h"]]
-  [["flang/Lower/AbstractConverter.h"]]
-  [["flang/Lower/IterationSpace.h"]]
-  [["flang/Lower/CallInterface.h"]]
-  [["flang/Lower/BoxAnalyzer.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/DirectivesCommon.h"]]
-)
diff --git a/flang/lib/Parser/CMakeLists.txt b/flang/lib/Parser/CMakeLists.txt
index 20c6c2a7c8f80..e0479b0da3eb8 100644
--- a/flang/lib/Parser/CMakeLists.txt
+++ b/flang/lib/Parser/CMakeLists.txt
@@ -25,6 +25,14 @@ add_flang_library(FortranParser
   unparse.cpp
   user-state.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/provenance.h"]]
+  [["flang/Parser/message.h"]]
+  [["flang/Parser/parse-tree-visitor.h"]]
+
   LINK_LIBS
   FortranSupport
 
@@ -37,11 +45,3 @@ add_flang_library(FortranParser
   omp_gen
   acc_gen
 )
-
-target_precompile_headers(FortranParser PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/provenance.h"]]
-  [["flang/Parser/message.h"]]
-  [["flang/Parser/parse-tree-visitor.h"]]
-)
diff --git a/flang/lib/Semantics/CMakeLists.txt b/flang/lib/Semantics/CMakeLists.txt
index 109bc2dbb8569..44e6dfb4dd09f 100644
--- a/flang/lib/Semantics/CMakeLists.txt
+++ b/flang/lib/Semantics/CMakeLists.txt
@@ -53,6 +53,15 @@ add_flang_library(FortranSemantics
   type.cpp
   unparse-with-symbols.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Semantics/semantics.h"]]
+  [["flang/Semantics/type.h"]]
+  [["flang/Semantics/openmp-modifiers.h"]]
+  [["flang/Semantics/expression.h"]]
+  [["flang/Semantics/tools.h"]]
+  [["flang/Semantics/symbol.h"]]
+
   DEPENDS
   acc_gen
   omp_gen
@@ -68,12 +77,3 @@ add_flang_library(FortranSemantics
   FrontendOpenACC
   TargetParser
 )
-
-target_precompile_headers(FortranSemantics PRIVATE
-  [["flang/Semantics/semantics.h"]]
-  [["flang/Semantics/type.h"]]
-  [["flang/Semantics/openmp-modifiers.h"]]
-  [["flang/Semantics/expression.h"]]
-  [["flang/Semantics/tools.h"]]
-  [["flang/Semantics/symbol.h"]]
-)
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 9954053591d30..a539a2182efa5 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -313,6 +313,22 @@ if(LLVM_CCACHE_BUILD)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
+
+      if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        # ccache with PCH can lead to false-positives when only a macro
+        # definition changes with non-Clang compilers, because macro definitions
+        # are not compared in preprocessed mode.
+        # See: https://github.com/ccache/ccache/issues/1668
+        if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(NOTICE "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported. CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON. "
+            "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override this.")
+          set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+        elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(WARNING "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported.")
+        endif()
+      endif()
     else()
       # Until a way to reliably configure ccache on Windows is found,
       # disable precompiled headers for Windows + ccache builds
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index d938214f9d0df..c9ed03d98cc9c 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -79,6 +79,48 @@ function(llvm_update_compile_flags name)
   target_compile_definitions(${name} PRIVATE ${LLVM_COMPILE_DEFINITIONS})
 endfunction()
 
+function(llvm_update_pch name)
+  if(LLVM_REQUIRES_RTTI OR LLVM_REQUIRES_EH)
+    # Non-default RTTI/EH results in incompatible flags, precluding PCH reuse.
+    set(ARG_DISABLE_PCH_REUSE ON)
+  endif()
+
+  # Find PCH with highest priority from dependencies. We reuse the first PCH
+  # with the highest priority. If the target has its own set of PCH, we give it
+  # a higher priority so that dependents will prefer the new PCH. We don't do
+  # transitive PCH reuse, because this causes too many unrelated naming
+  # collisions (e.g., in A -> B -> C{pch}, only B would reuse the PCH of C).
+  set(pch_priority 0)
+  llvm_map_components_to_libnames(libs
+    ${LLVM_LINK_COMPONENTS}
+  )
+  list(APPEND libs ${ARG_LINK_LIBS})
+  foreach(lib ${libs})
+    if(TARGET ${lib})
+      get_target_property(lib_pch_priority ${lib} LLVM_PCH_PRIORITY)
+      if(${lib_pch_priority} GREATER ${pch_priority})
+        set(pch_priority ${lib_pch_priority})
+        set(pch_reuse ${lib})
+      endif()
+    endif()
+  endforeach()
+
+  if(ARG_PRECOMPILE_HEADERS)
+    message(DEBUG "Adding PCH ${ARG_PRECOMPILE_HEADERS} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} PRIVATE $<$<COMPILE_LANGUAGE:CXX>:${ARG_PRECOMPILE_HEADERS}>)
+    if(NOT ARG_DISABLE_PCH_REUSE)
+      # Set priority so that dependants can reuse the PCH.
+      math(EXPR pch_priority "${pch_priority} + 1")
+      set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
+    endif()
+  elseif(pch_reuse AND NOT ARG_DISABLE_PCH_REUSE)
+    message(DEBUG "Using PCH ${pch_reuse} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} REUSE_FROM ${pch_reuse})
+  else()
+    message(DEBUG "Using NO PCH for ${name}")
+  endif()
+endfunction()
+
 function(add_llvm_symbol_exports target_name export_file)
   if("${CMAKE_SYSTEM_NAME}" MATCHES "Darwin")
     set(native_export_file "${target_name}.exports")
@@ -485,6 +527,13 @@ endfunction(set_windows_version_resource_properties)
 #     Corresponds to OUTPUT_NAME in target properties.
 #   DEPENDS targets...
 #     Same semantics as add_dependencies().
+#   PRECOMPILE_HEADERS include_directives...
+#     Pre-compiled C++ headers to use. PCH can be reused by dependants. If
+#     specified, no PCHs from dependencies will be reused.
+#   DISABLE_PCH_REUSE
+#     Disable reuse of pre-compiled headers in both directions: the library will
+#     not reuse the PCH of a dependency and a defined PCH will not be offered
+#     for reuse by dependants.
 #   LINK_COMPONENTS components...
 #     Same as the variable LLVM_LINK_COMPONENTS.
 #   LINK_LIBS lib_targets...
@@ -504,11 +553,12 @@ endfunction(set_windows_version_resource_properties)
 #   )
 function(llvm_add_library name)
   cmake_parse_arguments(ARG
-    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB"
+    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB;DISABLE_PCH_REUSE"
     "OUTPUT_NAME;PLUGIN_TOOL;ENTITLEMENTS;BUNDLE_PATH"
-    "ADDITIONAL_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
+    "ADDITIONAL_HEADERS;PRECOMPILE_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
     ${ARGN})
   list(APPEND LLVM_COMMON_DEPENDS ${ARG_DEPENDS})
+  list(APPEND LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS})
   if(ARG_ADDITIONAL_HEADERS)
     # Pass through ADDITIONAL_HEADERS.
     set(ARG_ADDITIONAL_HEADERS ADDITIONAL_HEADERS ${ARG_ADDITIONAL_HEADERS})
@@ -550,6 +600,7 @@ function(llvm_add_library name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     if(CMAKE_GENERATOR STREQUAL "Xcode")
       set(DUMMY_FILE ${CMAKE_CURRENT_BINARY_DIR}/Dummy.c)
       file(WRITE ${DUMMY_FILE} "// This file intentionally empty\n")
@@ -604,7 +655,7 @@ function(llvm_add_library name)
       ${output_name}
       OBJLIBS ${ALL_FILES} # objlib
       LINK_LIBS ${ARG_LINK_LIBS}
-      LINK_COMPONENTS ${ARG_LINK_COMPONENTS}
+      LINK_COMPONENTS ${LLVM_LINK_COMPONENTS}
       )
     set_target_properties(${name_static} PROPERTIES FOLDER "${subproject_title}/Libraries")
 
@@ -676,6 +727,11 @@ function(llvm_add_library name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT obj_name)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
   add_link_opts( ${name} )
   if(ARG_OUTPUT_NAME)
@@ -758,8 +814,7 @@ function(llvm_add_library name)
         target_compile_definitions(${name} PRIVATE LLVM_BUILD_STATIC)
       endif()
       llvm_map_components_to_libnames(llvm_libs
-       ${ARG_LINK_COMPONENTS}
-       ${LLVM_LINK_COMPONENTS}
+        ${LLVM_LINK_COMPONENTS}
        )
     endif()
   else()
@@ -770,7 +825,7 @@ function(llvm_add_library name)
     # It would be nice to verify that we have the dependencies for this library
     # name, but using get_property(... SET) doesn't suffice to determine if a
     # property has been set to an empty value.
-    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS} ${LLVM_LINK_COMPONENTS})
+    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${LLVM_LINK_COMPONENTS})
 
     # This property is an internal property only used to make sure the
     # link step applied in LLVMBuildResolveComponentsLink uses the same
@@ -993,6 +1048,7 @@ macro(generate_llvm_objects name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>")
     if(ARG_DEPENDS)
       add_dependencies(${obj_name} ${ARG_DEPENDS})
@@ -1032,7 +1088,7 @@ endmacro()
 
 macro(add_llvm_executable name)
   cmake_parse_arguments(ARG
-    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS"
+    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS;DISABLE_PCH_REUSE"
     "ENTITLEMENTS;BUNDLE_PATH"
     ""
     ${ARGN})
@@ -1073,6 +1129,11 @@ macro(add_llvm_executable name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
 
   if (ARG_SUPPORT_PLUGINS AND NOT "${CMAKE_SYSTEM_NAME}" MATCHES "AIX")
@@ -1780,7 +1841,7 @@ function(add_unittest test_suite test_name)
   endif()
 
   list(APPEND LLVM_LINK_COMPONENTS Support) # gtest needs it for raw_ostream
-  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH ${ARGN})
+  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH DISABLE_PCH_REUSE ${ARGN})
   get_subproject_title(subproject_title)
   set_target_properties(${test_name} PROPERTIES FOLDER "${subproject_title}/Tests/Unit")
 
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 311123084bf58..2e90c0ad0ad76 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -1304,6 +1304,27 @@ if (LLVM_BUILD_INSTRUMENTED AND LLVM_BUILD_INSTRUMENTED_COVERAGE)
   message(FATAL_ERROR "LLVM_BUILD_INSTRUMENTED and LLVM_BUILD_INSTRUMENTED_COVERAGE cannot both be specified")
 endif()
 
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  # Pre-compiled headers with GCC (tested versions 14+15) provide very little
+  # compile-time improvements, but substantially increase the build dir size.
+  # Therefore, disable PCH with GCC by default.
+  message(NOTICE "Precompiled headers are disabled by default with GCC. "
+    "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override.")
+  set(CMAKE_DISABLE_PRECOMPILE_HEADERS ON)
+endif()
+if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  message(STATUS "Precompiled headers enabled.")
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    # Clang requires this flag in order for precompiled headers to work with ccache
+    append("-Xclang -fno-pch-timestamp" CMAKE_CXX_FLAGS)
+  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # GCC requires this flag in order for precompiled headers to work with ccache
+    append("-fpch-preprocess" CMAKE_CXX_FLAGS)
+  endif()
+else()
+  message(STATUS "Precompiled headers disabled.")
+endif()
+
 set(LLVM_THINLTO_CACHE_PATH "${PROJECT_BINARY_DIR}/lto.cache" CACHE STRING "Set ThinLTO cache path. This can be used when building LLVM from several different directiories.")
 
 if(LLVM_ENABLE_LTO AND WIN32 AND NOT LINKER_IS_LLD_LINK AND NOT MINGW)
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
index cc50112f326ea..9a82cc0ed0916 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsAddObjectFile
   OrcV2CBindingsAddObjectFile.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
index 0f18d6c24912f..1a738f60d15e1 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsBasicUsage
   OrcV2CBindingsBasicUsage.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
index 8e2c97d782062..c7f748a12bf35 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsDumpObjects
   OrcV2CBindingsDumpObjects.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
index af1b43e43e111..b152faf1e3527 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
@@ -13,4 +13,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsIRTransforms
   OrcV2CBindingsIRTransforms.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
index 52eb2d496fc23..c46700d9b2576 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsLazy
   OrcV2CBindingsLazy.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
index 4cdc2b114e9a5..d4a06104120c8 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_exampl...
[truncated]

llvmbot · 2026-01-16T16:20:37Z

@llvm/pr-subscribers-testing-tools

Author: Alexis Engelke (aengelke)

Changes

This patch implements PCH support.

Libraries can defined precompiled headers using a newly added PRECOMPILE_HEADERS keyword. If specified, the listed headers will be compiled into a pre-compiled header using standard CMake mechanisms.
Libraries that don't define their own PRECOMPILE_HEADERS but directly depend on a library or component that defines its own PCH will reuse that PCH. This reuse is not transitive to prevent excessive use of unrelated headers. If multiple dependencies provide a reusable PCH, the first one with the longest dependency chain (stored in the CMake target property LLVM_PCH_PRIORITY) is used. However, due to CMake limitations, only PCH from targets that are already defined can be reused; therefore libraries that should reuse a PCH must be defined later in the CMake file (=> add_subdirectory order matters).
Libraries and executables can prevent PCH reuse with the keyword DISABLE_PCH_REUSE. This both prevents reuse from dependencies and reuse by other dependants. This is useful when, e.g., internal headers are used in the PCH or the used headers are unlikely to provide benefits for dependants.
Precompiled headers are only used for C++ targets. Due to CMake-weirdness, C-only targets must explicitly disable PCH reuse, otherwise configuration fails.
With GCC, PCH provide very little benefits (tested with GCC 14 and 15) due to increased template instatiation costs, but substantially increase max-rss and build directory size. Therefore, disable PCH with GCC by default; this can be explicitly overridden on the command line with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF.
With ccache and non-Clang compilers, changes in macro definitions are not always accurately forwarded with ccache's preprocessed mode. To be on the safe side, when ccache is enabled, disable PCH with all non-Clang compilers; this can be explicitly overridden.
Add a base PCH to LLVMSupport, which includes widely used standard library and Support+ADT headers. The pch.h is placed in include so that later PCH headers can extend that list of headers.
Flang PCH use is ported to the general mechanism.

Addition of PCH headers for other components (e.g., IR, CodeGen) will be posted as separate PRs.

RFC: https://discourse.llvm.org/t/rfc-use-pre-compiled-headers-to-speed-up-llvm-build-by-1-5-2x/89345

Patch is 25.98 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/176420.diff

25 Files Affected:

(modified) clang/tools/c-index-test/CMakeLists.txt (+2)
(modified) clang/tools/clang-fuzzer/dictionary/CMakeLists.txt (+2)
(modified) flang/CMakeLists.txt (-10)
(modified) flang/lib/Evaluate/CMakeLists.txt (+9-9)
(modified) flang/lib/Frontend/CMakeLists.txt (+8-8)
(modified) flang/lib/Lower/CMakeLists.txt (+11-11)
(modified) flang/lib/Parser/CMakeLists.txt (+8-8)
(modified) flang/lib/Semantics/CMakeLists.txt (+9-9)
(modified) llvm/CMakeLists.txt (+16)
(modified) llvm/cmake/modules/AddLLVM.cmake (+69-8)
(modified) llvm/cmake/modules/HandleLLVMOptions.cmake (+21)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsRemovableCode/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsVeryLazy/CMakeLists.txt (+2)
(added) llvm/include/llvm/Support/pch.h (+75)
(modified) llvm/lib/CMakeLists.txt (+1-1)
(modified) llvm/lib/Support/CMakeLists.txt (+6-1)
(modified) llvm/tools/llvm-c-test/CMakeLists.txt (+2)
(modified) llvm/utils/count/CMakeLists.txt (+2)
(modified) mlir/test/CAPI/CMakeLists.txt (+1)

diff --git a/clang/tools/c-index-test/CMakeLists.txt b/clang/tools/c-index-test/CMakeLists.txt
index 41e80e66ffa7a..2bbd387d6e812 100644
--- a/clang/tools/c-index-test/CMakeLists.txt
+++ b/clang/tools/c-index-test/CMakeLists.txt
@@ -5,6 +5,8 @@ set(LLVM_LINK_COMPONENTS
 add_clang_executable(c-index-test
   c-index-test.c
   core_main.cpp
+
+  DISABLE_PCH_REUSE # Prevent CMake error with C source files.
   )
 
 if(NOT MSVC)
diff --git a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
index 6b72b98f5e1c4..a7f18965b4e29 100644
--- a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
+++ b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
@@ -1,5 +1,7 @@
 set(CMAKE_CXX_FLAGS ${CXX_FLAGS_NOFUZZ})
 add_clang_executable(clang-fuzzer-dictionary
   dictionary.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
 
diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index c01eb56d5e496..e21304d2e4da7 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -441,11 +441,6 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   if (BUILD_SHARED_LIBS AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "Clang"))
    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-semantic-interposition")
   endif()
-
-  # GCC requires this flag in order for precompiled headers to work with ccache
-  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpch-preprocess")
-  endif()
 endif()
 
 # Clang on Darwin enables non-POSIX extensions by default, which allows the
@@ -456,11 +451,6 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
-# Clang requires this flag in order for precompiled headers to work with ccache
-if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
-endif()
-
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.
diff --git a/flang/lib/Evaluate/CMakeLists.txt b/flang/lib/Evaluate/CMakeLists.txt
index 24a1c9004bc3b..472ecb6d8d079 100644
--- a/flang/lib/Evaluate/CMakeLists.txt
+++ b/flang/lib/Evaluate/CMakeLists.txt
@@ -66,15 +66,8 @@ add_flang_library(FortranEvaluate
   ${LIBPGMATH}
   ${QUADMATHLIB}
 
-  LINK_COMPONENTS
-  Support
-
-  DEPENDS
-  acc_gen
-  omp_gen
-)
-
-target_precompile_headers(FortranEvaluate PRIVATE
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
   [["flang/Evaluate/common.h"]]
   [["flang/Evaluate/call.h"]]
   [["flang/Evaluate/traverse.h"]]
@@ -86,4 +79,11 @@ target_precompile_headers(FortranEvaluate PRIVATE
   [["flang/Evaluate/integer.h"]]
   [["flang/Evaluate/expression.h"]]
   [["flang/Evaluate/tools.h"]]
+
+  LINK_COMPONENTS
+  Support
+
+  DEPENDS
+  acc_gen
+  omp_gen
 )
diff --git a/flang/lib/Frontend/CMakeLists.txt b/flang/lib/Frontend/CMakeLists.txt
index 4ebe497e65676..bd67ec444f358 100644
--- a/flang/lib/Frontend/CMakeLists.txt
+++ b/flang/lib/Frontend/CMakeLists.txt
@@ -13,6 +13,14 @@ add_flang_library(flangFrontend
   TextDiagnosticBuffer.cpp
   TextDiagnostic.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/dump-parse-tree.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/Bridge.h"]]
+
   DEPENDS
   CUFDialect
   FIRDialect
@@ -78,11 +86,3 @@ add_flang_library(flangFrontend
   clangBasic
   clangOptions
 )
-
-target_precompile_headers(flangFrontend PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/dump-parse-tree.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/Bridge.h"]]
-)
diff --git a/flang/lib/Lower/CMakeLists.txt b/flang/lib/Lower/CMakeLists.txt
index 230a56ab66ec5..f5424c78b9a98 100644
--- a/flang/lib/Lower/CMakeLists.txt
+++ b/flang/lib/Lower/CMakeLists.txt
@@ -38,6 +38,17 @@ add_flang_library(FortranLower
   Support/Utils.cpp
   SymbolMap.cpp
   VectorSubscripts.cpp
+
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Lower/ConvertExpr.h"]]
+  [["flang/Lower/SymbolMap.h"]]
+  [["flang/Lower/AbstractConverter.h"]]
+  [["flang/Lower/IterationSpace.h"]]
+  [["flang/Lower/CallInterface.h"]]
+  [["flang/Lower/BoxAnalyzer.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/DirectivesCommon.h"]]
   
   DEPENDS
   CUFAttrs
@@ -79,14 +90,3 @@ add_flang_library(FortranLower
   MLIRLLVMDialect
   MLIRSCFToControlFlow
 )
-
-target_precompile_headers(FortranLower PRIVATE
-  [["flang/Lower/ConvertExpr.h"]]
-  [["flang/Lower/SymbolMap.h"]]
-  [["flang/Lower/AbstractConverter.h"]]
-  [["flang/Lower/IterationSpace.h"]]
-  [["flang/Lower/CallInterface.h"]]
-  [["flang/Lower/BoxAnalyzer.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/DirectivesCommon.h"]]
-)
diff --git a/flang/lib/Parser/CMakeLists.txt b/flang/lib/Parser/CMakeLists.txt
index 20c6c2a7c8f80..e0479b0da3eb8 100644
--- a/flang/lib/Parser/CMakeLists.txt
+++ b/flang/lib/Parser/CMakeLists.txt
@@ -25,6 +25,14 @@ add_flang_library(FortranParser
   unparse.cpp
   user-state.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/provenance.h"]]
+  [["flang/Parser/message.h"]]
+  [["flang/Parser/parse-tree-visitor.h"]]
+
   LINK_LIBS
   FortranSupport
 
@@ -37,11 +45,3 @@ add_flang_library(FortranParser
   omp_gen
   acc_gen
 )
-
-target_precompile_headers(FortranParser PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/provenance.h"]]
-  [["flang/Parser/message.h"]]
-  [["flang/Parser/parse-tree-visitor.h"]]
-)
diff --git a/flang/lib/Semantics/CMakeLists.txt b/flang/lib/Semantics/CMakeLists.txt
index 109bc2dbb8569..44e6dfb4dd09f 100644
--- a/flang/lib/Semantics/CMakeLists.txt
+++ b/flang/lib/Semantics/CMakeLists.txt
@@ -53,6 +53,15 @@ add_flang_library(FortranSemantics
   type.cpp
   unparse-with-symbols.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Semantics/semantics.h"]]
+  [["flang/Semantics/type.h"]]
+  [["flang/Semantics/openmp-modifiers.h"]]
+  [["flang/Semantics/expression.h"]]
+  [["flang/Semantics/tools.h"]]
+  [["flang/Semantics/symbol.h"]]
+
   DEPENDS
   acc_gen
   omp_gen
@@ -68,12 +77,3 @@ add_flang_library(FortranSemantics
   FrontendOpenACC
   TargetParser
 )
-
-target_precompile_headers(FortranSemantics PRIVATE
-  [["flang/Semantics/semantics.h"]]
-  [["flang/Semantics/type.h"]]
-  [["flang/Semantics/openmp-modifiers.h"]]
-  [["flang/Semantics/expression.h"]]
-  [["flang/Semantics/tools.h"]]
-  [["flang/Semantics/symbol.h"]]
-)
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 9954053591d30..a539a2182efa5 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -313,6 +313,22 @@ if(LLVM_CCACHE_BUILD)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
+
+      if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        # ccache with PCH can lead to false-positives when only a macro
+        # definition changes with non-Clang compilers, because macro definitions
+        # are not compared in preprocessed mode.
+        # See: https://github.com/ccache/ccache/issues/1668
+        if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(NOTICE "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported. CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON. "
+            "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override this.")
+          set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+        elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(WARNING "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported.")
+        endif()
+      endif()
     else()
       # Until a way to reliably configure ccache on Windows is found,
       # disable precompiled headers for Windows + ccache builds
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index d938214f9d0df..c9ed03d98cc9c 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -79,6 +79,48 @@ function(llvm_update_compile_flags name)
   target_compile_definitions(${name} PRIVATE ${LLVM_COMPILE_DEFINITIONS})
 endfunction()
 
+function(llvm_update_pch name)
+  if(LLVM_REQUIRES_RTTI OR LLVM_REQUIRES_EH)
+    # Non-default RTTI/EH results in incompatible flags, precluding PCH reuse.
+    set(ARG_DISABLE_PCH_REUSE ON)
+  endif()
+
+  # Find PCH with highest priority from dependencies. We reuse the first PCH
+  # with the highest priority. If the target has its own set of PCH, we give it
+  # a higher priority so that dependents will prefer the new PCH. We don't do
+  # transitive PCH reuse, because this causes too many unrelated naming
+  # collisions (e.g., in A -> B -> C{pch}, only B would reuse the PCH of C).
+  set(pch_priority 0)
+  llvm_map_components_to_libnames(libs
+    ${LLVM_LINK_COMPONENTS}
+  )
+  list(APPEND libs ${ARG_LINK_LIBS})
+  foreach(lib ${libs})
+    if(TARGET ${lib})
+      get_target_property(lib_pch_priority ${lib} LLVM_PCH_PRIORITY)
+      if(${lib_pch_priority} GREATER ${pch_priority})
+        set(pch_priority ${lib_pch_priority})
+        set(pch_reuse ${lib})
+      endif()
+    endif()
+  endforeach()
+
+  if(ARG_PRECOMPILE_HEADERS)
+    message(DEBUG "Adding PCH ${ARG_PRECOMPILE_HEADERS} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} PRIVATE $<$<COMPILE_LANGUAGE:CXX>:${ARG_PRECOMPILE_HEADERS}>)
+    if(NOT ARG_DISABLE_PCH_REUSE)
+      # Set priority so that dependants can reuse the PCH.
+      math(EXPR pch_priority "${pch_priority} + 1")
+      set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
+    endif()
+  elseif(pch_reuse AND NOT ARG_DISABLE_PCH_REUSE)
+    message(DEBUG "Using PCH ${pch_reuse} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} REUSE_FROM ${pch_reuse})
+  else()
+    message(DEBUG "Using NO PCH for ${name}")
+  endif()
+endfunction()
+
 function(add_llvm_symbol_exports target_name export_file)
   if("${CMAKE_SYSTEM_NAME}" MATCHES "Darwin")
     set(native_export_file "${target_name}.exports")
@@ -485,6 +527,13 @@ endfunction(set_windows_version_resource_properties)
 #     Corresponds to OUTPUT_NAME in target properties.
 #   DEPENDS targets...
 #     Same semantics as add_dependencies().
+#   PRECOMPILE_HEADERS include_directives...
+#     Pre-compiled C++ headers to use. PCH can be reused by dependants. If
+#     specified, no PCHs from dependencies will be reused.
+#   DISABLE_PCH_REUSE
+#     Disable reuse of pre-compiled headers in both directions: the library will
+#     not reuse the PCH of a dependency and a defined PCH will not be offered
+#     for reuse by dependants.
 #   LINK_COMPONENTS components...
 #     Same as the variable LLVM_LINK_COMPONENTS.
 #   LINK_LIBS lib_targets...
@@ -504,11 +553,12 @@ endfunction(set_windows_version_resource_properties)
 #   )
 function(llvm_add_library name)
   cmake_parse_arguments(ARG
-    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB"
+    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB;DISABLE_PCH_REUSE"
     "OUTPUT_NAME;PLUGIN_TOOL;ENTITLEMENTS;BUNDLE_PATH"
-    "ADDITIONAL_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
+    "ADDITIONAL_HEADERS;PRECOMPILE_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
     ${ARGN})
   list(APPEND LLVM_COMMON_DEPENDS ${ARG_DEPENDS})
+  list(APPEND LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS})
   if(ARG_ADDITIONAL_HEADERS)
     # Pass through ADDITIONAL_HEADERS.
     set(ARG_ADDITIONAL_HEADERS ADDITIONAL_HEADERS ${ARG_ADDITIONAL_HEADERS})
@@ -550,6 +600,7 @@ function(llvm_add_library name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     if(CMAKE_GENERATOR STREQUAL "Xcode")
       set(DUMMY_FILE ${CMAKE_CURRENT_BINARY_DIR}/Dummy.c)
       file(WRITE ${DUMMY_FILE} "// This file intentionally empty\n")
@@ -604,7 +655,7 @@ function(llvm_add_library name)
       ${output_name}
       OBJLIBS ${ALL_FILES} # objlib
       LINK_LIBS ${ARG_LINK_LIBS}
-      LINK_COMPONENTS ${ARG_LINK_COMPONENTS}
+      LINK_COMPONENTS ${LLVM_LINK_COMPONENTS}
       )
     set_target_properties(${name_static} PROPERTIES FOLDER "${subproject_title}/Libraries")
 
@@ -676,6 +727,11 @@ function(llvm_add_library name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT obj_name)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
   add_link_opts( ${name} )
   if(ARG_OUTPUT_NAME)
@@ -758,8 +814,7 @@ function(llvm_add_library name)
         target_compile_definitions(${name} PRIVATE LLVM_BUILD_STATIC)
       endif()
       llvm_map_components_to_libnames(llvm_libs
-       ${ARG_LINK_COMPONENTS}
-       ${LLVM_LINK_COMPONENTS}
+        ${LLVM_LINK_COMPONENTS}
        )
     endif()
   else()
@@ -770,7 +825,7 @@ function(llvm_add_library name)
     # It would be nice to verify that we have the dependencies for this library
     # name, but using get_property(... SET) doesn't suffice to determine if a
     # property has been set to an empty value.
-    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS} ${LLVM_LINK_COMPONENTS})
+    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${LLVM_LINK_COMPONENTS})
 
     # This property is an internal property only used to make sure the
     # link step applied in LLVMBuildResolveComponentsLink uses the same
@@ -993,6 +1048,7 @@ macro(generate_llvm_objects name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>")
     if(ARG_DEPENDS)
       add_dependencies(${obj_name} ${ARG_DEPENDS})
@@ -1032,7 +1088,7 @@ endmacro()
 
 macro(add_llvm_executable name)
   cmake_parse_arguments(ARG
-    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS"
+    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS;DISABLE_PCH_REUSE"
     "ENTITLEMENTS;BUNDLE_PATH"
     ""
     ${ARGN})
@@ -1073,6 +1129,11 @@ macro(add_llvm_executable name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
 
   if (ARG_SUPPORT_PLUGINS AND NOT "${CMAKE_SYSTEM_NAME}" MATCHES "AIX")
@@ -1780,7 +1841,7 @@ function(add_unittest test_suite test_name)
   endif()
 
   list(APPEND LLVM_LINK_COMPONENTS Support) # gtest needs it for raw_ostream
-  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH ${ARGN})
+  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH DISABLE_PCH_REUSE ${ARGN})
   get_subproject_title(subproject_title)
   set_target_properties(${test_name} PROPERTIES FOLDER "${subproject_title}/Tests/Unit")
 
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 311123084bf58..2e90c0ad0ad76 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -1304,6 +1304,27 @@ if (LLVM_BUILD_INSTRUMENTED AND LLVM_BUILD_INSTRUMENTED_COVERAGE)
   message(FATAL_ERROR "LLVM_BUILD_INSTRUMENTED and LLVM_BUILD_INSTRUMENTED_COVERAGE cannot both be specified")
 endif()
 
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  # Pre-compiled headers with GCC (tested versions 14+15) provide very little
+  # compile-time improvements, but substantially increase the build dir size.
+  # Therefore, disable PCH with GCC by default.
+  message(NOTICE "Precompiled headers are disabled by default with GCC. "
+    "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override.")
+  set(CMAKE_DISABLE_PRECOMPILE_HEADERS ON)
+endif()
+if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  message(STATUS "Precompiled headers enabled.")
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    # Clang requires this flag in order for precompiled headers to work with ccache
+    append("-Xclang -fno-pch-timestamp" CMAKE_CXX_FLAGS)
+  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # GCC requires this flag in order for precompiled headers to work with ccache
+    append("-fpch-preprocess" CMAKE_CXX_FLAGS)
+  endif()
+else()
+  message(STATUS "Precompiled headers disabled.")
+endif()
+
 set(LLVM_THINLTO_CACHE_PATH "${PROJECT_BINARY_DIR}/lto.cache" CACHE STRING "Set ThinLTO cache path. This can be used when building LLVM from several different directiories.")
 
 if(LLVM_ENABLE_LTO AND WIN32 AND NOT LINKER_IS_LLD_LINK AND NOT MINGW)
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
index cc50112f326ea..9a82cc0ed0916 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsAddObjectFile
   OrcV2CBindingsAddObjectFile.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
index 0f18d6c24912f..1a738f60d15e1 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsBasicUsage
   OrcV2CBindingsBasicUsage.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
index 8e2c97d782062..c7f748a12bf35 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsDumpObjects
   OrcV2CBindingsDumpObjects.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
index af1b43e43e111..b152faf1e3527 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
@@ -13,4 +13,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsIRTransforms
   OrcV2CBindingsIRTransforms.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
index 52eb2d496fc23..c46700d9b2576 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsLazy
   OrcV2CBindingsLazy.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
index 4cdc2b114e9a5..d4a06104120c8 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_exampl...
[truncated]

llvmbot · 2026-01-16T16:20:37Z

@llvm/pr-subscribers-flang-fir-hlfir

Author: Alexis Engelke (aengelke)

Changes

This patch implements PCH support.

Libraries can defined precompiled headers using a newly added PRECOMPILE_HEADERS keyword. If specified, the listed headers will be compiled into a pre-compiled header using standard CMake mechanisms.
Libraries that don't define their own PRECOMPILE_HEADERS but directly depend on a library or component that defines its own PCH will reuse that PCH. This reuse is not transitive to prevent excessive use of unrelated headers. If multiple dependencies provide a reusable PCH, the first one with the longest dependency chain (stored in the CMake target property LLVM_PCH_PRIORITY) is used. However, due to CMake limitations, only PCH from targets that are already defined can be reused; therefore libraries that should reuse a PCH must be defined later in the CMake file (=> add_subdirectory order matters).
Libraries and executables can prevent PCH reuse with the keyword DISABLE_PCH_REUSE. This both prevents reuse from dependencies and reuse by other dependants. This is useful when, e.g., internal headers are used in the PCH or the used headers are unlikely to provide benefits for dependants.
Precompiled headers are only used for C++ targets. Due to CMake-weirdness, C-only targets must explicitly disable PCH reuse, otherwise configuration fails.
With GCC, PCH provide very little benefits (tested with GCC 14 and 15) due to increased template instatiation costs, but substantially increase max-rss and build directory size. Therefore, disable PCH with GCC by default; this can be explicitly overridden on the command line with -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF.
With ccache and non-Clang compilers, changes in macro definitions are not always accurately forwarded with ccache's preprocessed mode. To be on the safe side, when ccache is enabled, disable PCH with all non-Clang compilers; this can be explicitly overridden.
Add a base PCH to LLVMSupport, which includes widely used standard library and Support+ADT headers. The pch.h is placed in include so that later PCH headers can extend that list of headers.
Flang PCH use is ported to the general mechanism.

Addition of PCH headers for other components (e.g., IR, CodeGen) will be posted as separate PRs.

RFC: https://discourse.llvm.org/t/rfc-use-pre-compiled-headers-to-speed-up-llvm-build-by-1-5-2x/89345

Patch is 25.98 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/176420.diff

25 Files Affected:

(modified) clang/tools/c-index-test/CMakeLists.txt (+2)
(modified) clang/tools/clang-fuzzer/dictionary/CMakeLists.txt (+2)
(modified) flang/CMakeLists.txt (-10)
(modified) flang/lib/Evaluate/CMakeLists.txt (+9-9)
(modified) flang/lib/Frontend/CMakeLists.txt (+8-8)
(modified) flang/lib/Lower/CMakeLists.txt (+11-11)
(modified) flang/lib/Parser/CMakeLists.txt (+8-8)
(modified) flang/lib/Semantics/CMakeLists.txt (+9-9)
(modified) llvm/CMakeLists.txt (+16)
(modified) llvm/cmake/modules/AddLLVM.cmake (+69-8)
(modified) llvm/cmake/modules/HandleLLVMOptions.cmake (+21)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsRemovableCode/CMakeLists.txt (+2)
(modified) llvm/examples/OrcV2Examples/OrcV2CBindingsVeryLazy/CMakeLists.txt (+2)
(added) llvm/include/llvm/Support/pch.h (+75)
(modified) llvm/lib/CMakeLists.txt (+1-1)
(modified) llvm/lib/Support/CMakeLists.txt (+6-1)
(modified) llvm/tools/llvm-c-test/CMakeLists.txt (+2)
(modified) llvm/utils/count/CMakeLists.txt (+2)
(modified) mlir/test/CAPI/CMakeLists.txt (+1)

diff --git a/clang/tools/c-index-test/CMakeLists.txt b/clang/tools/c-index-test/CMakeLists.txt
index 41e80e66ffa7a..2bbd387d6e812 100644
--- a/clang/tools/c-index-test/CMakeLists.txt
+++ b/clang/tools/c-index-test/CMakeLists.txt
@@ -5,6 +5,8 @@ set(LLVM_LINK_COMPONENTS
 add_clang_executable(c-index-test
   c-index-test.c
   core_main.cpp
+
+  DISABLE_PCH_REUSE # Prevent CMake error with C source files.
   )
 
 if(NOT MSVC)
diff --git a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
index 6b72b98f5e1c4..a7f18965b4e29 100644
--- a/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
+++ b/clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
@@ -1,5 +1,7 @@
 set(CMAKE_CXX_FLAGS ${CXX_FLAGS_NOFUZZ})
 add_clang_executable(clang-fuzzer-dictionary
   dictionary.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
 
diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index c01eb56d5e496..e21304d2e4da7 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -441,11 +441,6 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   if (BUILD_SHARED_LIBS AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "Clang"))
    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-semantic-interposition")
   endif()
-
-  # GCC requires this flag in order for precompiled headers to work with ccache
-  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpch-preprocess")
-  endif()
 endif()
 
 # Clang on Darwin enables non-POSIX extensions by default, which allows the
@@ -456,11 +451,6 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
-# Clang requires this flag in order for precompiled headers to work with ccache
-if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
-endif()
-
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.
diff --git a/flang/lib/Evaluate/CMakeLists.txt b/flang/lib/Evaluate/CMakeLists.txt
index 24a1c9004bc3b..472ecb6d8d079 100644
--- a/flang/lib/Evaluate/CMakeLists.txt
+++ b/flang/lib/Evaluate/CMakeLists.txt
@@ -66,15 +66,8 @@ add_flang_library(FortranEvaluate
   ${LIBPGMATH}
   ${QUADMATHLIB}
 
-  LINK_COMPONENTS
-  Support
-
-  DEPENDS
-  acc_gen
-  omp_gen
-)
-
-target_precompile_headers(FortranEvaluate PRIVATE
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
   [["flang/Evaluate/common.h"]]
   [["flang/Evaluate/call.h"]]
   [["flang/Evaluate/traverse.h"]]
@@ -86,4 +79,11 @@ target_precompile_headers(FortranEvaluate PRIVATE
   [["flang/Evaluate/integer.h"]]
   [["flang/Evaluate/expression.h"]]
   [["flang/Evaluate/tools.h"]]
+
+  LINK_COMPONENTS
+  Support
+
+  DEPENDS
+  acc_gen
+  omp_gen
 )
diff --git a/flang/lib/Frontend/CMakeLists.txt b/flang/lib/Frontend/CMakeLists.txt
index 4ebe497e65676..bd67ec444f358 100644
--- a/flang/lib/Frontend/CMakeLists.txt
+++ b/flang/lib/Frontend/CMakeLists.txt
@@ -13,6 +13,14 @@ add_flang_library(flangFrontend
   TextDiagnosticBuffer.cpp
   TextDiagnostic.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/dump-parse-tree.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/Bridge.h"]]
+
   DEPENDS
   CUFDialect
   FIRDialect
@@ -78,11 +86,3 @@ add_flang_library(flangFrontend
   clangBasic
   clangOptions
 )
-
-target_precompile_headers(flangFrontend PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/dump-parse-tree.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/Bridge.h"]]
-)
diff --git a/flang/lib/Lower/CMakeLists.txt b/flang/lib/Lower/CMakeLists.txt
index 230a56ab66ec5..f5424c78b9a98 100644
--- a/flang/lib/Lower/CMakeLists.txt
+++ b/flang/lib/Lower/CMakeLists.txt
@@ -38,6 +38,17 @@ add_flang_library(FortranLower
   Support/Utils.cpp
   SymbolMap.cpp
   VectorSubscripts.cpp
+
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Lower/ConvertExpr.h"]]
+  [["flang/Lower/SymbolMap.h"]]
+  [["flang/Lower/AbstractConverter.h"]]
+  [["flang/Lower/IterationSpace.h"]]
+  [["flang/Lower/CallInterface.h"]]
+  [["flang/Lower/BoxAnalyzer.h"]]
+  [["flang/Lower/PFTBuilder.h"]]
+  [["flang/Lower/DirectivesCommon.h"]]
   
   DEPENDS
   CUFAttrs
@@ -79,14 +90,3 @@ add_flang_library(FortranLower
   MLIRLLVMDialect
   MLIRSCFToControlFlow
 )
-
-target_precompile_headers(FortranLower PRIVATE
-  [["flang/Lower/ConvertExpr.h"]]
-  [["flang/Lower/SymbolMap.h"]]
-  [["flang/Lower/AbstractConverter.h"]]
-  [["flang/Lower/IterationSpace.h"]]
-  [["flang/Lower/CallInterface.h"]]
-  [["flang/Lower/BoxAnalyzer.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/DirectivesCommon.h"]]
-)
diff --git a/flang/lib/Parser/CMakeLists.txt b/flang/lib/Parser/CMakeLists.txt
index 20c6c2a7c8f80..e0479b0da3eb8 100644
--- a/flang/lib/Parser/CMakeLists.txt
+++ b/flang/lib/Parser/CMakeLists.txt
@@ -25,6 +25,14 @@ add_flang_library(FortranParser
   unparse.cpp
   user-state.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Parser/parsing.h"]]
+  [["flang/Parser/parse-tree.h"]]
+  [["flang/Parser/provenance.h"]]
+  [["flang/Parser/message.h"]]
+  [["flang/Parser/parse-tree-visitor.h"]]
+
   LINK_LIBS
   FortranSupport
 
@@ -37,11 +45,3 @@ add_flang_library(FortranParser
   omp_gen
   acc_gen
 )
-
-target_precompile_headers(FortranParser PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/provenance.h"]]
-  [["flang/Parser/message.h"]]
-  [["flang/Parser/parse-tree-visitor.h"]]
-)
diff --git a/flang/lib/Semantics/CMakeLists.txt b/flang/lib/Semantics/CMakeLists.txt
index 109bc2dbb8569..44e6dfb4dd09f 100644
--- a/flang/lib/Semantics/CMakeLists.txt
+++ b/flang/lib/Semantics/CMakeLists.txt
@@ -53,6 +53,15 @@ add_flang_library(FortranSemantics
   type.cpp
   unparse-with-symbols.cpp
 
+  DISABLE_PCH_REUSE
+  PRECOMPILE_HEADERS
+  [["flang/Semantics/semantics.h"]]
+  [["flang/Semantics/type.h"]]
+  [["flang/Semantics/openmp-modifiers.h"]]
+  [["flang/Semantics/expression.h"]]
+  [["flang/Semantics/tools.h"]]
+  [["flang/Semantics/symbol.h"]]
+
   DEPENDS
   acc_gen
   omp_gen
@@ -68,12 +77,3 @@ add_flang_library(FortranSemantics
   FrontendOpenACC
   TargetParser
 )
-
-target_precompile_headers(FortranSemantics PRIVATE
-  [["flang/Semantics/semantics.h"]]
-  [["flang/Semantics/type.h"]]
-  [["flang/Semantics/openmp-modifiers.h"]]
-  [["flang/Semantics/expression.h"]]
-  [["flang/Semantics/tools.h"]]
-  [["flang/Semantics/symbol.h"]]
-)
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 9954053591d30..a539a2182efa5 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -313,6 +313,22 @@ if(LLVM_CCACHE_BUILD)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
+
+      if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        # ccache with PCH can lead to false-positives when only a macro
+        # definition changes with non-Clang compilers, because macro definitions
+        # are not compared in preprocessed mode.
+        # See: https://github.com/ccache/ccache/issues/1668
+        if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(NOTICE "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported. CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON. "
+            "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override this.")
+          set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+        elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+          message(WARNING "Using ccache with precompiled headers with non-Clang "
+            "compilers is not supported.")
+        endif()
+      endif()
     else()
       # Until a way to reliably configure ccache on Windows is found,
       # disable precompiled headers for Windows + ccache builds
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index d938214f9d0df..c9ed03d98cc9c 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -79,6 +79,48 @@ function(llvm_update_compile_flags name)
   target_compile_definitions(${name} PRIVATE ${LLVM_COMPILE_DEFINITIONS})
 endfunction()
 
+function(llvm_update_pch name)
+  if(LLVM_REQUIRES_RTTI OR LLVM_REQUIRES_EH)
+    # Non-default RTTI/EH results in incompatible flags, precluding PCH reuse.
+    set(ARG_DISABLE_PCH_REUSE ON)
+  endif()
+
+  # Find PCH with highest priority from dependencies. We reuse the first PCH
+  # with the highest priority. If the target has its own set of PCH, we give it
+  # a higher priority so that dependents will prefer the new PCH. We don't do
+  # transitive PCH reuse, because this causes too many unrelated naming
+  # collisions (e.g., in A -> B -> C{pch}, only B would reuse the PCH of C).
+  set(pch_priority 0)
+  llvm_map_components_to_libnames(libs
+    ${LLVM_LINK_COMPONENTS}
+  )
+  list(APPEND libs ${ARG_LINK_LIBS})
+  foreach(lib ${libs})
+    if(TARGET ${lib})
+      get_target_property(lib_pch_priority ${lib} LLVM_PCH_PRIORITY)
+      if(${lib_pch_priority} GREATER ${pch_priority})
+        set(pch_priority ${lib_pch_priority})
+        set(pch_reuse ${lib})
+      endif()
+    endif()
+  endforeach()
+
+  if(ARG_PRECOMPILE_HEADERS)
+    message(DEBUG "Adding PCH ${ARG_PRECOMPILE_HEADERS} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} PRIVATE $<$<COMPILE_LANGUAGE:CXX>:${ARG_PRECOMPILE_HEADERS}>)
+    if(NOT ARG_DISABLE_PCH_REUSE)
+      # Set priority so that dependants can reuse the PCH.
+      math(EXPR pch_priority "${pch_priority} + 1")
+      set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
+    endif()
+  elseif(pch_reuse AND NOT ARG_DISABLE_PCH_REUSE)
+    message(DEBUG "Using PCH ${pch_reuse} for ${name} (prio ${pch_priority})")
+    target_precompile_headers(${name} REUSE_FROM ${pch_reuse})
+  else()
+    message(DEBUG "Using NO PCH for ${name}")
+  endif()
+endfunction()
+
 function(add_llvm_symbol_exports target_name export_file)
   if("${CMAKE_SYSTEM_NAME}" MATCHES "Darwin")
     set(native_export_file "${target_name}.exports")
@@ -485,6 +527,13 @@ endfunction(set_windows_version_resource_properties)
 #     Corresponds to OUTPUT_NAME in target properties.
 #   DEPENDS targets...
 #     Same semantics as add_dependencies().
+#   PRECOMPILE_HEADERS include_directives...
+#     Pre-compiled C++ headers to use. PCH can be reused by dependants. If
+#     specified, no PCHs from dependencies will be reused.
+#   DISABLE_PCH_REUSE
+#     Disable reuse of pre-compiled headers in both directions: the library will
+#     not reuse the PCH of a dependency and a defined PCH will not be offered
+#     for reuse by dependants.
 #   LINK_COMPONENTS components...
 #     Same as the variable LLVM_LINK_COMPONENTS.
 #   LINK_LIBS lib_targets...
@@ -504,11 +553,12 @@ endfunction(set_windows_version_resource_properties)
 #   )
 function(llvm_add_library name)
   cmake_parse_arguments(ARG
-    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB"
+    "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME;NO_INSTALL_RPATH;COMPONENT_LIB;DISABLE_PCH_REUSE"
     "OUTPUT_NAME;PLUGIN_TOOL;ENTITLEMENTS;BUNDLE_PATH"
-    "ADDITIONAL_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
+    "ADDITIONAL_HEADERS;PRECOMPILE_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
     ${ARGN})
   list(APPEND LLVM_COMMON_DEPENDS ${ARG_DEPENDS})
+  list(APPEND LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS})
   if(ARG_ADDITIONAL_HEADERS)
     # Pass through ADDITIONAL_HEADERS.
     set(ARG_ADDITIONAL_HEADERS ADDITIONAL_HEADERS ${ARG_ADDITIONAL_HEADERS})
@@ -550,6 +600,7 @@ function(llvm_add_library name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     if(CMAKE_GENERATOR STREQUAL "Xcode")
       set(DUMMY_FILE ${CMAKE_CURRENT_BINARY_DIR}/Dummy.c)
       file(WRITE ${DUMMY_FILE} "// This file intentionally empty\n")
@@ -604,7 +655,7 @@ function(llvm_add_library name)
       ${output_name}
       OBJLIBS ${ALL_FILES} # objlib
       LINK_LIBS ${ARG_LINK_LIBS}
-      LINK_COMPONENTS ${ARG_LINK_COMPONENTS}
+      LINK_COMPONENTS ${LLVM_LINK_COMPONENTS}
       )
     set_target_properties(${name_static} PROPERTIES FOLDER "${subproject_title}/Libraries")
 
@@ -676,6 +727,11 @@ function(llvm_add_library name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT obj_name)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
   add_link_opts( ${name} )
   if(ARG_OUTPUT_NAME)
@@ -758,8 +814,7 @@ function(llvm_add_library name)
         target_compile_definitions(${name} PRIVATE LLVM_BUILD_STATIC)
       endif()
       llvm_map_components_to_libnames(llvm_libs
-       ${ARG_LINK_COMPONENTS}
-       ${LLVM_LINK_COMPONENTS}
+        ${LLVM_LINK_COMPONENTS}
        )
     endif()
   else()
@@ -770,7 +825,7 @@ function(llvm_add_library name)
     # It would be nice to verify that we have the dependencies for this library
     # name, but using get_property(... SET) doesn't suffice to determine if a
     # property has been set to an empty value.
-    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS} ${LLVM_LINK_COMPONENTS})
+    set_property(TARGET ${name} PROPERTY LLVM_LINK_COMPONENTS ${LLVM_LINK_COMPONENTS})
 
     # This property is an internal property only used to make sure the
     # link step applied in LLVMBuildResolveComponentsLink uses the same
@@ -993,6 +1048,7 @@ macro(generate_llvm_objects name)
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
+    llvm_update_pch(${obj_name})
     set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>")
     if(ARG_DEPENDS)
       add_dependencies(${obj_name} ${ARG_DEPENDS})
@@ -1032,7 +1088,7 @@ endmacro()
 
 macro(add_llvm_executable name)
   cmake_parse_arguments(ARG
-    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS"
+    "DISABLE_LLVM_LINK_LLVM_DYLIB;IGNORE_EXTERNALIZE_DEBUGINFO;NO_INSTALL_RPATH;SUPPORT_PLUGINS;EXPORT_SYMBOLS;DISABLE_PCH_REUSE"
     "ENTITLEMENTS;BUNDLE_PATH"
     ""
     ${ARGN})
@@ -1073,6 +1129,11 @@ macro(add_llvm_executable name)
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)
     llvm_update_compile_flags(${name})
+    llvm_update_pch(${name})
+  else()
+    target_precompile_headers(${name} REUSE_FROM ${obj_name})
+    get_target_property(pch_priority ${obj_name} LLVM_PCH_PRIORITY)
+    set_target_properties(${name} PROPERTIES LLVM_PCH_PRIORITY ${pch_priority})
   endif()
 
   if (ARG_SUPPORT_PLUGINS AND NOT "${CMAKE_SYSTEM_NAME}" MATCHES "AIX")
@@ -1780,7 +1841,7 @@ function(add_unittest test_suite test_name)
   endif()
 
   list(APPEND LLVM_LINK_COMPONENTS Support) # gtest needs it for raw_ostream
-  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH ${ARGN})
+  add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH DISABLE_PCH_REUSE ${ARGN})
   get_subproject_title(subproject_title)
   set_target_properties(${test_name} PROPERTIES FOLDER "${subproject_title}/Tests/Unit")
 
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 311123084bf58..2e90c0ad0ad76 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -1304,6 +1304,27 @@ if (LLVM_BUILD_INSTRUMENTED AND LLVM_BUILD_INSTRUMENTED_COVERAGE)
   message(FATAL_ERROR "LLVM_BUILD_INSTRUMENTED and LLVM_BUILD_INSTRUMENTED_COVERAGE cannot both be specified")
 endif()
 
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  # Pre-compiled headers with GCC (tested versions 14+15) provide very little
+  # compile-time improvements, but substantially increase the build dir size.
+  # Therefore, disable PCH with GCC by default.
+  message(NOTICE "Precompiled headers are disabled by default with GCC. "
+    "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override.")
+  set(CMAKE_DISABLE_PRECOMPILE_HEADERS ON)
+endif()
+if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  message(STATUS "Precompiled headers enabled.")
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    # Clang requires this flag in order for precompiled headers to work with ccache
+    append("-Xclang -fno-pch-timestamp" CMAKE_CXX_FLAGS)
+  elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # GCC requires this flag in order for precompiled headers to work with ccache
+    append("-fpch-preprocess" CMAKE_CXX_FLAGS)
+  endif()
+else()
+  message(STATUS "Precompiled headers disabled.")
+endif()
+
 set(LLVM_THINLTO_CACHE_PATH "${PROJECT_BINARY_DIR}/lto.cache" CACHE STRING "Set ThinLTO cache path. This can be used when building LLVM from several different directiories.")
 
 if(LLVM_ENABLE_LTO AND WIN32 AND NOT LINKER_IS_LLD_LINK AND NOT MINGW)
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
index cc50112f326ea..9a82cc0ed0916 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsAddObjectFile
   OrcV2CBindingsAddObjectFile.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
index 0f18d6c24912f..1a738f60d15e1 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsBasicUsage
   OrcV2CBindingsBasicUsage.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
index 8e2c97d782062..c7f748a12bf35 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsDumpObjects/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsDumpObjects
   OrcV2CBindingsDumpObjects.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
index af1b43e43e111..b152faf1e3527 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsIRTransforms/CMakeLists.txt
@@ -13,4 +13,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsIRTransforms
   OrcV2CBindingsIRTransforms.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
index 52eb2d496fc23..c46700d9b2576 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsLazy/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_example(OrcV2CBindingsLazy
   OrcV2CBindingsLazy.c
+
+  DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
   )
diff --git a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
index 4cdc2b114e9a5..d4a06104120c8 100644
--- a/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
+++ b/llvm/examples/OrcV2Examples/OrcV2CBindingsMCJITLikeMemoryManager/CMakeLists.txt
@@ -12,4 +12,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_exampl...
[truncated]

aengelke · 2026-01-16T16:25:58Z

@boomanaiden154 Regarding the pre-commit CI: sccache appears to not support PCH and I think it would be a good idea to disable PCH in the pre-commit CI to catch missing includes. I have no idea where the configuration lives and what I would need to do to pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON, can you give some pointers here?

The PR currently disables PCH for GCC, which means that post-commit builders will implicitly have coverage for both PCH and non-PCH builds.

aengelke · 2026-01-16T16:59:39Z

c-t-t, in total -15% instructions:u/wall-time on stage2-clang.

boomanaiden154 · 2026-01-16T17:35:16Z

boomanaiden154 Regarding the pre-commit CI: sccache appears to not support PCH and I think it would be a good idea to disable PCH in the pre-commit CI to catch missing includes. I have no idea where the configuration lives and what I would need to do to pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON, can you give some pointers here?

I thought we already had it disabled, but it looks like we do not. The config is here

llvm-project/.ci/monolithic-linux.sh

Line 42 in 8dd848e

cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \

.

There's also a monolithic-windows.sh on the Windows side that you might need to touch given we use clang-cl over there, but not exactly sure how that interacts.

vbvictor · 2026-01-16T17:53:31Z

Regarding the pre-commit CI: sccache appears to not support PCH and I think it would be a good idea to disable PCH in the pre-commit CI to catch missing includes.

Can we instead set CMAKE_DISABLE_PRECOMPILE_HEADERS=ON inside llvm/CMakeLists.txt if we see that
CMAKE_CXX_COMPILER_LAUNCHER is set too sccache?
I use sccache locally, and currently, I will need to set DISABLE_PRECOMPILE_HEADERS manually, right? I think we should give a warning message if sccache + pch is activated.

makslevental · 2026-01-16T18:27:38Z

Regarding the pre-commit CI: sccache appears to not support PCH and I think it would be a good idea to disable PCH in the pre-commit CI to catch missing includes.

Can we instead set CMAKE_DISABLE_PRECOMPILE_HEADERS=ON inside llvm/CMakeLists.txt if we see that CMAKE_CXX_COMPILER_LAUNCHER is set too sccache? I use sccache locally, and currently, I will need to set DISABLE_PRECOMPILE_HEADERS manually, right? I think we should give a warning message if sccache + pch is activated.

I would prefer not to make this automatic - we have enough "hand-holding/automatic" such variables which cause problems eventually because they wholesale force some behavior for all targets and all users. A warning is fine.

clang/tools/c-index-test/CMakeLists.txt

flang/lib/Frontend/CMakeLists.txt

makslevental

Just my 2 cents: The CMake-isms look completely reasonable but I'll come back to take a closer look soon. But thank you for working on this - a %1 improvement in LLVM compile time probably saves 1000 trees per year or something like that 😄.

llvm/examples/OrcV2Examples/OrcV2CBindingsAddObjectFile/CMakeLists.txt

llvm/cmake/modules/HandleLLVMOptions.cmake

kuhar · 2026-01-16T20:32:24Z

llvm/include/llvm/Support/pch.h

+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Twine.h"


How did you decide on these exact includes? I think MathExtras and SmallVectorExtras are also popular.

I looked at the output of -ftime-trace of a full Clang+LLVM build, sorted by time spent in parsing (including transitively included headers), and selected headers that showed up high in that list. This is obviously not accurate, but was sufficient for the initial RFC. That's also how headers like JSON ended up in the list: it isn't used much, but it takes ~180ms to parse+instantiate templates (although I'm unsure whether it's a net win to have it in the PCH).

The includes of Clang+LLVM by counts (cut off at 100, output by ninja -t deps) and current PCH status is below. The numbers can, however, be slightly misleading: llvm/ADT/ilist_node.h is used 2631 times, but in >=2205 cases, this will be pulled in by Instruction.h, which will get part of the llvm/IR PCH. (NB: MathExtras is currently transitively included, SmallVectorExtras is only used in 6 CUs in Clang+LLVM.)

We can certainly adjust the list, consider this to be a not-particularly-optimized initial proposal. I'm not sure if we want to invest a lot of time in tooling to determine an ideal PCH set, however, as it is a very complex and multi-dimensional optimization problem.

5322 PCH llvm/Support/Compiler.h 5280 PCH llvm/ADT/STLForwardCompat.h 5275 PCH llvm/ADT/STLFunctionalExtras.h 5275 PCH llvm/ADT/DenseMapInfo.h 5275 PCH llvm/ADT/ADL.h 5273 PCH llvm/ADT/iterator_range.h 5265 PCH llvm/Support/DataTypes.h 5251 PCH llvm/ADT/SmallVector.h 5250 PCH llvm/Support/ErrorHandling.h 5245 PCH llvm/ADT/bit.h 5239 PCH llvm/Support/type_traits.h 5232 PCH llvm/ADT/StringRef.h 5229 PCH llvm/Support/SwapByteOrder.h 5228 PCH llvm/ADT/Hashing.h 5220 PCH llvm/ADT/iterator.h 5218 PCH llvm/ADT/STLExtras.h 5121 PCH llvm/Support/MathExtras.h 5103 PCH llvm/Support/raw_ostream.h 5048 PCH llvm/ADT/ArrayRef.h 4960 PCH llvm/Support/MemAlloc.h 4925 PCH llvm/Support/PointerLikeTypeTraits.h 4822 PCH llvm/Support/ReverseIteration.h 4821 PCH llvm/ADT/EpochTracker.h 4815 PCH llvm/Support/AlignOf.h 4797 PCH llvm/ADT/Twine.h 4797 PCH llvm/ADT/DenseMap.h 4660 PCH llvm/ADT/SmallString.h 4572 PCH llvm/Support/Alignment.h 4557 PCH llvm/Support/ErrorOr.h 4536 PCH llvm/Support/Debug.h 4506 PCH llvm/Support/Format.h 4490 PCH llvm/Support/AllocatorBase.h 4402 PCH llvm/Support/Error.h 4381 PCH llvm/Support/Casting.h 4229 PCH llvm/ADT/StringMapEntry.h 4218 PCH llvm/Support/float128.h 4218 PCH llvm/ADT/APInt.h 4195 PCH llvm/ADT/StringMap.h 4102 PCH llvm/Support/Allocator.h 4093 PCH llvm/ADT/StringSwitch.h 4036 PCH llvm/ADT/BitmaskEnum.h 4004 PCH llvm/ADT/DenseSet.h 3893 PCH llvm/Support/CBindingWrapping.h 3870 llvm/ADT/PointerIntPair.h 3850 PCH llvm/Support/VersionTuple.h 3811 PCH llvm/ADT/APSInt.h 3670 PCH llvm/ADT/StringExtras.h 3587 llvm/ADT/PointerUnion.h 3479 llvm/Support/CodeGen.h 3393 PCH llvm/ADT/SmallPtrSet.h 3231 PCH llvm/ADT/FloatingPointMode.h 3214 llvm/Support/TypeSize.h 3205 llvm/ADT/Sequence.h 3125 PCH llvm/ADT/BitVector.h 3110 PCH llvm/Support/Endian.h 3085 llvm/Support/TrailingObjects.h 3052 PCH llvm/Support/SMLoc.h 3051 llvm/Support/AtomicOrdering.h 3050 llvm/ADT/MapVector.h 2913 PCH llvm/ADT/APFloat.h 2865 PCH llvm/Support/MemoryBufferRef.h 2833 llvm/ADT/TinyPtrVector.h 2812 PCH llvm/Support/MemoryBuffer.h 2807 PCH llvm/Support/StringSaver.h 2760 PCH llvm/ADT/IntrusiveRefCntPtr.h 2675 PCH llvm/ADT/SetVector.h 2640 llvm/ADT/StringTable.h 2633 llvm/ADT/ilist_node_base.h 2631 llvm/ADT/ilist_node_options.h 2631 llvm/ADT/ilist_node.h 2621 llvm/ADT/ilist_base.h 2620 llvm/ADT/simple_ilist.h 2620 llvm/ADT/ilist_iterator.h 2604 llvm/Support/Compression.h 2593 llvm/ADT/ilist.h 2501 llvm/ADT/GraphTraits.h 2480 llvm/ADT/FunctionExtras.h 2383 PCH llvm/Support/CommandLine.h 2370 PCH llvm/Support/FormatVariadicDetails.h 2289 llvm/Support/xxhash.h 2273 PCH llvm/Support/SourceMgr.h 2272 llvm/ADT/Bitfields.h 2247 llvm/Support/ModRef.h 2206 llvm/ADT/StringSet.h 2201 llvm/Support/Threading.h 2105 PCH llvm/Support/MD5.h 2105 llvm/ADT/FoldingSet.h 1939 llvm/Support/TypeName.h 1777 llvm/Support/RWMutex.h 1763 PCH llvm/Support/FileSystem/UniqueID.h 1753 PCH llvm/Support/NativeFormatting.h 1748 PCH llvm/Support/FormatProviders.h 1601 llvm/Support/Printable.h 1590 llvm/Support/Recycler.h 1570 llvm/Support/BranchProbability.h 1557 llvm/Support/Discriminator.h 1529 llvm/ADT/SmallSet.h 1395 PCH llvm/Support/Chrono.h 1386 PCH llvm/Support/FileSystem.h 1299 llvm/Support/UniqueBBID.h 1293 llvm/Support/Path.h 1276 llvm/Support/RecyclingAllocator.h 1260 llvm/ADT/SparseBitVector.h 1210 llvm/Support/TargetOpcodes.def 1209 llvm/ADT/PointerSumType.h 1207 llvm/Support/ArrayRecycler.h 1189 llvm/Support/DXILABI.h 1171 llvm/Support/HashBuilder.h 1151 llvm/Support/AllocToken.h 1109 llvm/Support/Errc.h 1078 llvm/ADT/CachedHashString.h 1074 llvm/Support/ExtensibleRTTI.h 1040 llvm/Support/PrettyStackTrace.h 1038 llvm/ADT/Uniformity.h 1011 llvm/Support/raw_os_ostream.h 956 llvm/ADT/IndexedMap.h 951 llvm/Support/AMDGPUAddrSpace.h 951 llvm/ADT/GenericSSAContext.h 942 llvm/ADT/GenericCycleInfo.h 939 llvm/Support/PGOOptions.h 916 llvm/Support/Mutex.h 916 llvm/ADT/APFixedPoint.h 899 llvm/Support/VirtualFileSystem.h 885 llvm/ADT/StableHashing.h 842 llvm/ADT/SmallBitVector.h 804 llvm/ADT/Bitset.h 801 llvm/Support/Regex.h 788 llvm/Support/KnownFPClass.h 756 llvm/ADT/DepthFirstIterator.h 717 llvm/ADT/PagedVector.h 694 llvm/Support/GenericDomTree.h 694 llvm/Support/CFGUpdate.h 694 llvm/Support/CFGDiff.h 693 llvm/Support/DynamicLibrary.h 688 llvm/Support/BlockFrequency.h 613 PCH llvm/Support/FormatVariadic.h 613 PCH llvm/Support/FormatCommon.h 597 llvm/Support/Registry.h 552 llvm/Support/Hash.h 476 llvm/ADT/PostOrderIterator.h 471 llvm/ADT/IntervalMap.h 450 PCH llvm/Support/YAMLParser.h 443 llvm/Support/ConvertUTF.h 434 PCH llvm/Support/YAMLTraits.h 433 PCH llvm/ADT/Statistic.h 414 llvm/Support/FormattedStream.h 398 llvm/ADT/SetOperations.h 395 llvm/Support/ELFAttributes.h 393 llvm/Support/BinaryStreamError.h 392 llvm/Support/BinaryStream.h 378 llvm/Support/GenericLoopInfo.h 353 llvm/Support/KnownBits.h 340 llvm/Support/InstructionCost.h 340 llvm/Support/BinaryStreamRef.h 335 llvm/Support/BinaryStreamArray.h 332 llvm/Support/BinaryStreamReader.h 321 llvm/Support/ARMBuildAttributes.h 320 llvm/Support/WithColor.h 308 llvm/Support/GenericLoopInfoImpl.h 301 llvm/Support/Timer.h 285 llvm/Support/SaveAndRestore.h 273 llvm/Support/FileCollector.h 270 llvm/Support/Signals.h 268 llvm/Support/VirtualOutputConfig.h 268 llvm/Support/VirtualOutputConfig.def 266 llvm/Support/VirtualOutputFile.h 264 llvm/Support/VirtualOutputBackend.h 262 llvm/Support/BuryPointer.h 257 llvm/Support/DataExtractor.h 252 llvm/Support/Program.h 243 PCH llvm/Support/JSON.h 232 llvm/Support/BinaryStreamWriter.h 217 llvm/ADT/ImmutableSet.h 213 llvm/Support/Memory.h 206 llvm/ADT/ImmutableMap.h 202 llvm/ADT/ImmutableList.h 186 llvm/Support/MSVCErrorWorkarounds.h 185 llvm/Support/TargetSelect.h 184 llvm/ADT/Any.h 172 llvm/Testing/Support/SupportHelpers.h 170 llvm/Support/EndianStream.h 169 llvm/Support/FileOutputBuffer.h 164 llvm/Support/ManagedStatic.h 163 llvm/Support/LEB128.h 157 llvm/ADT/SparseSet.h 155 llvm/Support/Process.h 154 llvm/Support/ScopedPrinter.h 150 llvm/Support/BinaryByteStream.h 150 llvm/ADT/fallible_iterator.h 149 llvm/Testing/Support/Error.h 147 llvm/ADT/EquivalenceClasses.h 107 llvm/Support/CheckedArithmetic.h 103 llvm/ADT/SparseMultiSet.h 100 llvm/ADT/IntEqClasses.h

You could try using ClangBuildAnalyzer
which simply analyzes the files created when compiling w/ -ftime-trace, and reports the top N longest-to-parse headers, the top N most included headers, the top N longest-to-instatiate templates, etc.

Apparently, it also by default only includes the root-included headers in the report. That is to say, by default, it excludes header inclusions that were first included by another header.
This may help with figuring out which headers should be prioritized over others.

aengelke · 2026-01-17T09:02:00Z

I think we should give a warning message if sccache + pch is activated.

Added warnings for typical CMAKE_CXX_COMPILER_LAUNCHER values.

how do we enable PCH? Is there some standard cmake flag or an llvm-specific one? Could you include this in the PR description?

Added a note that PCH is on-by-default unless noted as exception.

Err, how is it going to interact with downstream projects that don't disable PCH for C files?

If the downstream just uses standard CMake add_library etc., nothing changes. If the downstream uses AddLLVM (e.g., add_llvm_library) for a C-only target and doesn't specify DISABLE_PCH_REUSE, then CMake will fail with an error similar to:

CMake Error in examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt:
  Unable to resolve full path of PCH-header
  '/path/to/llvm/build/lib/Support/CMakeFiles/LLVMSupport.dir/cmake_pch.h'
  assigned to target OrcV2CBindingsBasicUsage, although its path is supposed
  to be known!

github-actions · 2026-01-17T10:05:39Z

🐧 Linux x64 Test Results

196330 tests passed
6419 tests skipped

✅ The build succeeded and all tests passed.

Spliced out from llvm#176420 to make sure that CI is fine without PCH.

aengelke · 2026-01-17T13:25:18Z

Re build failure: it turns out that CMake disables warnings in PCH by default, I filed #176566 to work around this.

nikic · 2026-01-17T14:31:46Z

Should I pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON on the compile time tracker so that we still become aware of expensive includes being added to core headers? (On the other hand, that makes it harder to tune the PCH configuration.)

nikic · 2026-01-17T14:39:57Z

Regarding the pre-commit CI: sccache appears to not support PCH and I think it would be a good idea to disable PCH in the pre-commit CI to catch missing includes.

Can we instead set CMAKE_DISABLE_PRECOMPILE_HEADERS=ON inside llvm/CMakeLists.txt if we see that CMAKE_CXX_COMPILER_LAUNCHER is set too sccache? I use sccache locally, and currently, I will need to set DISABLE_PRECOMPILE_HEADERS manually, right? I think we should give a warning message if sccache + pch is activated.

I would prefer not to make this automatic - we have enough "hand-holding/automatic" such variables which cause problems eventually because they wholesale force some behavior for all targets and all users. A warning is fine.

I'm unsure on this point. sccache is a common CI configuration, and nobody is going to notice those warnings. I guess this depends somewhat on what the failure mode for sccache + pch is. Build failures? Silent miscompiles? Silent failures to use the cache?

makslevental · 2026-01-17T16:21:54Z

I'm unsure on this point. sccache is a common CI configuration, and nobody is going to notice those warnings. I guess this depends somewhat on what the failure mode for sccache + pch is. Build failures? Silent miscompiles? Silent failures to use the cache?

Fair enough. I was thining about this and probably this one is fine because PCH either works on your system or doesn't (it's the CMAKE_MODULE_LINKER_FLAGS-type flags that are a problem where it should be determined on a target by target basis).

aengelke · 2026-01-17T16:23:36Z

Should I pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON on the compile time tracker so that we still become aware of expensive includes being added to core headers?

I'm unsure, but would be inclined not to do so for the reason you mentioned. Ideally, of course, we would track both, but increasing iteration times from ~15 to ~21 minutes is probably unacceptable (and I'd personally see them much rather at 11 minutes).

Maybe we can use data from stage1 somehow? In principle, this should be possible: for every ccache miss, track time+max-rss, and aggregate with data from ccache hits. Unfortunately, I don't think this is implementable without ccache changes.

Alternatively, collect data not for every commit? That would probably be an architectural change.

sccache is a common CI configuration, and nobody is going to notice those warnings. I guess this depends somewhat on what the failure mode for sccache + pch is. Build failures? Silent miscompiles? Silent failures to use the cache?

As with ccache: miscompiles with GCC, ok with Clang (in my limited testing):

sccache will incorrectly identify the PCH as cache hit even if a macro definition changed, due to identical preprocessor output. I filed PCH cause false postive cache hits mozilla/sccache#2562.
This causes miscompiles for GCC. (edit to clarify: this PR proposes to disable PCH with GCC by default in all configurations anyway.)
But the results for Clang appear to be correct and there is the expected number of cache hits from source files. It appears that Clang doesn't include macro definitions in the PCH? At least in my tests, the PCH ended up being identical (same hash sum)? (@ChuanqiXu9 @Bigcheese is this correct?)

Inside CMake, we cannot reliably whether ccache or sccache is actually used (e.g., clang could actually be /usr/lib/ccache/clang), so any automation here is going to be brittle to some degree.

llvm/cmake/modules/AddLLVM.cmake

Spliced out from #176420 to make sure that CI is fine without PCH, which are currently used by Flang.

Spliced out from llvm#176420 to make sure that CI is fine without PCH, which are currently used by Flang.

nikic

LGTM

llvm/cmake/modules/HandleLLVMOptions.cmake

nikic · 2026-01-26T10:19:55Z

clang/tools/clang-fuzzer/dictionary/CMakeLists.txt

  dictionary.c
+
+  # No C++ files, prevent CMake error.
+  DISABLE_PCH_REUSE


I wonder whether this is something we can detect and handle automatically?

bangtianliu · 2026-01-27T00:35:12Z

We cherry-picked this PR for IREE CI test: iree-org/iree#23279. It seems that you need to add DISABLE_PCH_REUSE to more cases to handle the incompatibilities. Refer to iree-org@f474998 for our fixes.

aengelke · 2026-01-27T15:50:31Z

@bangtianliu Thanks for testing. I fixed the combination with CMAKE_POSITION_INDEPENDENT_CODE=ON. I couldn't reproduce the error regarding visibility. Can you provide a LLVM CMake invocation that leads to this error?

sharkautarch · 2026-01-27T16:02:32Z

We cherry-picked this PR for IREE CI test: iree-org/iree#23279. It seems that you need to add DISABLE_PCH_REUSE to more cases to handle the incompatibilities. Refer to iree-org@f474998 for our fixes.

Instead of disabling pch for -fPIE executables, couldn’t you just add -fPIC to the flags?

bangtianliu · 2026-01-27T16:16:52Z

@bangtianliu Thanks for testing. I fixed the combination with CMAKE_POSITION_INDEPENDENT_CODE=ON. I couldn't reproduce the error regarding visibility. Can you provide a LLVM CMake invocation that leads to this error?

should be here: https://github.com/iree-org/llvm-project/blob/1839675d168e357c3db1559446031d55833f0181/llvm/lib/Target/CMakeLists.txt#L25-L32

so should be -DBUILD_SHARED_LIBS=OFF

aengelke · 2026-01-27T16:19:54Z

We already add -fPIC when LLVM_ENABLE_PIC is set (on by default). But CMAKE_POSITION_INDEPENDENT_CODE additionally adds -fPIE afterwards in a way that seems to be not suppressible via documented methods without making the resulting executable link without -pie. We could probably add -fPIC via target_compile_options instead of CMAKE_CXX_FLAGS, which should move it to the end, but this does seem very hacky... it would also break if LLVM_ENABLE_PIC is disabled.

@bangtianliu I mean a cmake command line invocation. BUILD_SHARED_LIBS is off by default. I can't reproduce a similar error on my system with Clang or GCC.

github-actions · 2026-01-27T16:33:58Z

🪟 Windows x64 Test Results

135761 tests passed
4450 tests skipped

✅ The build succeeded and all tests passed.

bangtianliu · 2026-01-27T17:49:29Z

@bangtianliu I mean a cmake command line invocation. BUILD_SHARED_LIBS is off by default. I can't reproduce a similar error on my system with Clang or GCC.

@aengelke
I successfully reproduced the visibility error. The key is using Clang-14.

  cmake -GNinja -B build -S llvm \
   -DCMAKE_C_COMPILER=/usr/bin/clang \      # Must be clang-14
   -DCMAKE_CXX_COMPILER=/usr/bin/clang++ \  # Must be clang++-14
   -DLLVM_TARGETS_TO_BUILD="X86;AArch64" \
   -DBUILD_SHARED_LIBS=OFF \
   -DCMAKE_BUILD_TYPE=Release

 # Build a Target library (triggers the error)
 ninja lib/Target/X86/CMakeFiles/LLVMX86CodeGen.dir/X86AsmPrinter.cpp.o

Refer to https://gist.github.com/bangtianliu/eb23d95e72cd7b9787c9f8736f13817a for the log

aengelke · 2026-01-27T18:23:56Z

Thanks for investigating. For me, Clang 17 is broken but Clang 18 works, so disabling for Clang <18 by default.

ChuanqiXu9 · 2026-01-28T04:57:13Z

But the results for Clang appear to be correct and there is the expected number of cache hits from source files. It appears that Clang doesn't include macro definitions in the PCH? At least in my tests, the PCH ended up being identical (same hash sum)? (@ChuanqiXu9 @Bigcheese is this correct?)

I think clang include the macro definition in the PCH. You can verify this by inserting outputs to ASTWriter::WritePreprocessor

aengelke requested review from dwblaikie, kuhar, mrkajetanp and nikic January 16, 2026 16:20

llvmbot added cmake Build system in general and CMake in particular flang:driver mlir flang Flang issues not falling into any other category llvm:support flang:fir-hlfir testing-tools flang:semantics flang:parser labels Jan 16, 2026

Disable PCH for Polly

b1c4558

aengelke mentioned this pull request Jan 16, 2026

[RFC] Use pre-compiled headers to speed up LLVM build (~1.5-2x) #173868

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[CI] Disable precompiled headers in pre-commit CI

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Spliced out from llvm#176420 to make sure that CI is fine without PCH.

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[CI] Disable precompiled headers in pre-commit CI #176563

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[CI] Disable precompiled headers in pre-commit CI (#176563)

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Spliced out from #176420 to make sure that CI is fine without PCH, which are currently used by Flang.

Priyanshu3820 pushed a commit to Priyanshu3820/llvm-project that referenced this pull request Jan 18, 2026

[CI] Disable precompiled headers in pre-commit CI (llvm#176563)

304d62e

Spliced out from llvm#176420 to make sure that CI is fine without PCH, which are currently used by Flang.

Merge remote-tracking branch 'origin/main' into pch-base

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[CI] Disable precompiled headers in pre-commit CI (llvm#176563)

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Spliced out from llvm#176420 to make sure that CI is fine without PCH, which are currently used by Flang.

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[TEST][DO NOT MERGE]Update llvm-project to include PCH infrastructure iree-org/iree#23279

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disable PCH reuse if CMake PIC is enabled

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disable PCH with Clang<18 and merge ccache/sccache warnings

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[CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH #176420

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[CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH #176420

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bangtianliu commented Jan 27, 2026 •

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github-actions bot commented Jan 27, 2026 •

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bangtianliu commented Jan 27, 2026 •

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